It's not that they don't turn up; they probably do. It's the constituencies of the two candidates. Hillary attracts the kind of person who doesn't go to rallies; who isn't particularly politically active. They show up every 4 years, vote for the name they know and then go on with their lives. Obama attracts the netroots and the grassroots; the kind of person who pays attention.

This is what explains both the difference in rally sizes and the non-correlation to the vote totals.