Democrats almost never care enough about their cronies to primary them out. Only rare upsets (Crowley) or defectors on important issues (Lipinski) get the boot. The Dems also never seem to retire, which is why their entire leadership class in Congress is approaching 80 or older. In general, only the Bernie Bro wing of the party seems to care enough to primary people and put pressure on their incumbents, and even organizations like the Justice Dems have been turned into a joke by supporting people like Neera Tanden.
The other factor in Democratic leadership is that committee spots for Democrats don't have term limits while the GOP does.
