Although "ain't" has always been popular with the lower classes of society, "rednecks" are certainly not to blame for the creation of the word. The predecessor of ain't or "a'nt" (a contraction of "am not" or "are not") was popularized by English playrights of the seventeenth century, likely in an effort to keep lines of the play within the restrictive syllablic quantity pushed on them by their's plays format.

tl;dr - people who likely had more class than you invented a word that you credit to people you consider to be beneath you.