People are always looking for fresh ways to express something
A foreign word may carry richer meaning or connotations than an English word (although that doesn't appear to be the case here). For example, a popular software package is named "LibreOffice" from the Spanish "libre", meaning "free." The English word "free" can mean "at no cost" or "without restrictions", but Spanish expresses these as "gratis" and "libre", respectively. The software creators presumably wanted to be clearer than "FreeOffice" would allow.
Language always conveys both meaning and cultural signals. An English speaker who says that a piece of art has "a certain je ne sais quoi" may feel that this sounds more cultured than "a certain indefinable quality". Similarly, an American who says "post hoc ergo propter hoc" in denouncing someone else's argument may be attempting to convey "I am a well-learned person, and I base my argument on ancient principles of logic".
English is a wholesale importer of words from other languages. Consequently it has a vast vocabulary, and speakers and writers of English appear to enjoy having different ways of saying the same thing, especially to convey precise shades of meaning.
In the case of kudos, it would not be the same to say glory or honor even though that is what the original Greek means. In English the word carries the connotation of praise, and is used as a declaration of acknowledgment for someone's achievements or actions.
There are many, many reasons as to why, but I'm only going to list some:
Settlement of England by various other people. When Normandy conquered the Anglo-Saxons, the language spoken by the Anglo-Saxons was Old English. However, words like "swine" got incorporated into the language.
Religion: When monks and priests came to the British Isles, they brought with them their Latin and Greek ancient texts. In time, some of the Greek and Latin words became part of the language as well. Just keep your eyes out for the next few days, and you will be able to spot those words. I can't seem to bring any to mind at the moment.
Those are the two most general reasons I can think of.
I think that sometimes prestige can orient our word choice. In some fields of communication specific words seem more prestigious than others, for example, French words are highly recommended in fields such as arts whereas Greek and Latin words give a sientific flavour to the speech of doctors and scientists.Moreover, upper middle and upper classes in society prefer to stuff their talks with foreign words, especially French, to highlight their prominence and power.