In addition to the other reasons mentioned, the media platformed him every chance they got. He was great for ratings and a good excuse for ignoring Bernie. He got an obscene amount of free coverage.
that was explicitly the strategy of the Clinton campaign. pied piper. email leaks confirmed that later. they figured if they could get a clown as the republican nomination it would be easy for Hillary to win.
If iirc the Republicans messed with the Democrats primaries and tricked them into choosing Mcgovern because he was the weakest Democrat candidate. Wallace was a huge POS who ended up getting shot in the neck later on.