I suspect many more have realized that unless they live in one of a half dozen or so states, their vote for Presidential (or even Senate) candidates is unlikely to affect the outcome.

For example, the state I live in (Kansas) has chosen the Republican candidate in every election in my lifetime, and in only one election has it been by a margin of less than 13% -- due to Ross Perot being a spoiler in 1992, where he took 27% of the state's vote.

My vote for President doesn't really matter, no matter which candidate I care to support, because I already know the outcome in my state. As such, back in the days when Presidential candidates both seemed to have the country's best interests at heart, even if I disagreed with one or both candidates' policies to do so, I generally didn't bother.