Truly good intelligent people can make more money, have less pressure and get more respect doing a hell of a lot of other things. A great deal of this came from paid lies from Republican donors starting when the Scaife Foundation and others were supporting American Spectator with large amounts of money to conduct attacks on the Clintons. I'm guessing you are aware of the birthers and Obama.We seem to be getting to the point where the only requirement for being president is that you are not obviously mentally compromised... Though you could argue that Trump is probably mentally compromised but in a different way than Biden.
When that becomes the case, and we don't seem to have standards, much less high standards, for our candidates, truly intelligent, good, quality people may decide not to run.