It seems that both "theism" and "deism" take their roots in Classical words for "god." Is there an etymological explanation for why they have such different meanings?
I wasn't able to find anything beyond the fact that "theism" is an earlier word whereas "deism," according to Wiktionary, comes from French.
Simplistically, deist (and hence deism) derives from Zeus while theist (and hence theism) derives from the more generic theos, both terms of Greek extraction:
deist (n.) 1620s, from French déiste, from Latin deus (see Zeus). Related: Deistic (1795). Also see deism. - etymonline
theist (n.) 1660s, from Greek theos "god" (from PIE root *dhes-, forming words for religious concepts) + -ist. The original senses was that later reserved to deist: "one who believes in a transcendent god but denies revelation." Later in 18c. theist was contrasted with deist, as believing in a personal God and allowing the possibility of revelation. - etymonline
The term deism goes back to the name Zeus, which in turn goes back to a PIE root meaning to shine. Further:
However, the word "deism", as it is understood today, is generally used to refer to the movement toward natural theology or freethinking that occurred in 17th-century Europe, and specifically in Britain. - wikipedia
It might be that 'theologians' (using that term loosely, given the distinction deists made with respect to theism) of the enlightenment movement (which championed reasoning) picked the term deism for its link to the notion of 'light' embedded in the movement's name The Enlightenment. There is also some (very slight) circumstantial support in the "father of lights" term attributed to someone said to be a deist:
Deist authors – and 17th- and 18th-century theologians in general – referred to God using a variety of vivid circumlocutions such as ... Father of Lights. Benjamin Franklin used this terminology when proposing that meetings of the Constitutional Convention begin with prayers - wikipedia
The term theist has a broader scope:
The term theism was first used by Ralph Cudworth (1617–1688).8 In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things". - wikipedia, citing Cudworth, Ralph (1678). The True Intellectual System of the Universe, Vol. I. New York: Gould & Newman, 1837, p. 267.
It would make sense that the distinction between deism and theism solidified due to the philosophical polarisation between the concepts of an involved God and that of one uninvolved:
Prior to the 17th Century the terms ["deism" and "deist"] were used interchangeably with the terms "theism" and "theist", respectively. ... Theologians and philosophers of the 17th Century began to give a different signification to the words... Both [theists and deists] asserted belief in one supreme God, the Creator... . But the theist taught that God remained actively interested in and operative in the world which he had made, whereas the Deist maintained that God endowed the world at creation with self-sustaining and self-acting powers and then surrendered it wholly to the operation of these powers acting as second causes. - wikipedia, citing: Orr, John (1934). English Deism: Its Roots and Its Fruits. Eerdmans. p. 13.
I don't think there is any etymological explanation of the difference in meaning and usage between "deism" and "theism". If there is one, it doesn't seem to be very obvious, as you have already noticed.
Different words often naturally develop different connotations, and when they don't, people often put them to use for different concepts or try to figure out some way to distinguish them (or sometimes, one of them just falls out of common use). I think this is probably the main reason why "theism" and "deism" have distinct meanings in present-day English.
"Deism" and "theism" both seem to have been coined sometime in the second half of the 2nd millenium. The Oxford English Dictionary (OED)'s earliest example of deism in English is from 1682; the OED further indicates that the word was taken from French. The French CNRTL indicates the earliest source where the French déisme is known to have been used is "1662 (Pasc., Pens., part. II, art. 4 ds Littré)". The French word for "God", dieu/Dieu, is related to the Latin stem used in déisme.
The OED's earliest example of theism in English is from 1678. It says théisme was used in French by Voltaire (who lived 1694–1778), but it doesn't say in what context. The French CNRTL says théisme was actually taken into French from English, and gives "1745 (Diderot, Principes de la philosophie morale, ou essai de M. S** A. Cooper Comte de [Shaftesbury] sur le mérite et la vertu, avec réflexions, Amsterdam, p. 12, note)" as the earliest known example in French.
Theism and deism are fairly "jargon"-y words. Jargon doesn't have to have a logical etymology; the point is to have any kind of label at all. E.g. the prefixes "a" and "im-" usually mean the same thing, but people have come up with a special distinction between "amoral" and "immoral".
So I would say there is no etymological explanation. The Oxford English Dictionary entry for "deism" cites a source that says that:
1877 E. R. Conder Basis of Faith i. 25 Deism should etymologically have the same sense with Theism, but it is commonly taken to carry with it the denial of what is called revealed religion. Theism conveys no such implication.
Deism is the understanding that a collection of natural laws govern existance. But either rejects the existence of supernatural beings, or simply rejects that God or gods take physical form to interact with humans, or that they interfere with the affairs of humans in a manner that causes a favorable or unfavorable outcome. Basically God is a collection of natural laws that govern everything.
Theology deals with gods and goddesses and includes monotheism (one God) which is distinctly different than a force or natural law but a being.
Deism proposed that an omnipotent being, if one existed, would be much too busy to be bothered by our petty daily complaints and desires, and furthormore, would be much too wise than to interfere on anyone's account; for one man's gain is another's loss. Mortal problems should be worked out amongst other mortals using reason, justice, and natural laws as a measure. For a God to intervene would break the law of free will.
Many of the founding fathers of the United States of America were deists and knew exactly the monumental thing they were trying to achieve. They wanted the declaration of Independence and later the Constitution and bill of eights to reflect the deist principles. "All men are created equal" directly challenges anyone who would decide to declare themselves a god, which happened all throughout history.
Early deists included many founding fathers and scientists, and deism was key to the development of modern science. If forces in nature govern the universe, it would make since to find out just what those forces are. Like Voltaire influenced the then Queen of Russia into funding science and allowing more free expression which caused Russia's golden era. In contrast, at the same time in Europe, deist thoughts had to be kept hidden in knights templar, Rosicrucian, and other secret orders.
Many deists find the Bible, Quran, Bhagavad-Gita, and other texts to be allegorical stories that teach society about natural law.
Much could be argued that ancients who were educated knew that the gods were in fact allegory describing the forces in natural law. The Egyptians believed strongly in "as above, so below" which was derived from observation of repeating patterns in nature. Not saying they weren't superstitious, but deism was around long before religions as we know it today.
Thomas Jefferson, George washington, Voltaire, many rebound early scientists. I mean it's the anyone who was anyone list.
