Second reason: The Democrats have a system. Anyone can't just ride in an expect to take a place. On the Republican side it's much easier for any yutz to enter the race and go from there.
But if there is ever a Republican incumbent again, it's possible he could run as a Democrat. The left will love him when he, "stands up," and doesn't back down to the media, corporations, the party establishment.
If Hillary loses in 2016, the Dems will run either Elizabeth Warren or Julian Castro in 2020. If Hillary wins in 2016, there will be no America in 2020 to run for.
“On the Republican side it’s much easier for any yutz to enter the race and go from there. “
If A yutz could just walk in and get 24%, what does that make the other candidates?