Tough question.

You could point to a lot of factors - economic, social, technological and such. Generally, though, times were good; politics didn't seem to matter all that much; the more level-headed people started drifting away to other walks of life. After all - why pursue a political career when the private sector offers far better alternatives for the talented individual? There even came a time where declaring in polite company that you were interested in politics would get you funny looks - sort of like getting caught picking your nose in public...
And just as the centrist majority gradually stopped caring, the lunatic fringe was waiting on the sidelines - like a primary tumor getting ready to metastasize. Perhaps things might have stayed that way, but then social media happened, and suddenly every John Q. Tinfoilhat was handed a metaphorical bullhorn and trotted out in front of a curious audience. The consequences are out there for all to see: the last 20 years have been marked by unchecked spread of the most harebrained conspiracy theories imaginable. Masses of people now believe in stuff that would make a seven-year-old cringe back in the day. Is it really a surprise that ruthless politicians came out of the woodwork to capitalize...?
Course, the above is just my personal opinion; YMMV.