How ‘master’ became ‘mister’ Grammarphobia
In late Middle English, people began using “Mr.,” an abbreviated
version of “master,” as a title “prefixed to the surname or first name
of a man without a higher, honorific, or professional title,”
according to the OED.
When people began speaking it, “Mr.” was pronounced like “master,” but “from the 16th cent. it was, at least in rapid or careless
speech, with consequent alteration of the vowel of the first
syllable,” according to the dictionary.
The first written example in the OED for the word spelled “mister” is from a 1642, and as the article notes it was a convoluted semantic shift of convenience, not one motivated by slave, master, Mr. etc. Even now, with master freed from mister, master can be used for juvenile men ... but its use now fades, probably for now extant political correctness.
So, why was Master weakened to Mister? It was simple semantic shift for brevity of abbreviations. There were dozens of different spellings for master. Brevity here meaning ... for lack of a better term, laziness and (economy) of language, not some early politically correct mid period BrE.