I am not sure this is a Ask Historians question as it seems more of a sociological question to me, but here is my guess.

Education for a very long time was only available to the rich and privileged. It used complex "big" words often with archaic meanings as a way to distance themselves from the common man. To the common man this was rightly taken as them being pretentious snobs. As education became available even to the poor the connotation of pretentiousness to that style of talking remained in the general populace. No one likes to feel that they are being talked down to.

This question has been removed for soapboxing. There's too much complaining about "American vocabulary today" for it to be a good question.