It doesn't make any sense to me. He is much stronger than any of his Republican challengers, yet over and over again I read in the comments sections here and elsewhere that he needs to get a primary to be brought back towards the Left. In my opinion he is on the Left but in keeping with his personal way of pragmatism he does what is necessary to keep things from getting worse. A primary challenge to "bring him back to the Left" is foolish to use against a President.
The Right uses the primary challenge in a smarter way: against Representatives and Senators. Why do you think the entire Republican party is shaking in it's boots every time somebody talks about the Tea Party? Because they know that if they step out of line they'll get a primary, simple as that. Do you think any elected Democratic Representative or Senator feels similar anxiety? It's because instead of trying to instigate primaries against Representatives and Senators who drift right-ward, Democrats look at their PRESIDENT and threaten primary him/her or to sit on their hands if he doesn't do what they want.
Everybody on the Left wanted a Public Option in the ACA, but it wasn't going to happen with the current composition of Congress at the time so President Obama went forward without it and brought us change. Most were grumbling and calling the President weak or saying "it was Rahm's fault we didn't get a Public Option". Similar arguments about the President's supposed weakness when he elected Richard Cordray instead of Elizabeth Warren to head the CFPB. Call it capitulation, spinelessness, whatever you want, but when you look at the whole picture President Obama usual has underwhelming, yet viable choices in the legislation he gets to sign or the nominees he gets to elect. President Obama is not an all-or-nothing leader, and to expect such isn't realistic.
I really shouldn't need to mention President Carter's fate at the hands of a strong Republican Challenger named Ronald Reagan after being savaged by Senator Ted Kennedy. That argument's been run into the ground, but the point is Senator Kennedy didn't help.
Progressives aren't going to get more of a say unless they help two things happen:
1) They could instigate primary challenges for Democratic Representives and Senators like the Tea Party did and continue to do to their Republican Representatives and Senators or just hold their nose and vote Straight D.
2) They help re-elect Barack Obama so he can change the 5-4 composition of the Supreme Court of the United States.
Only when we accomplish these tasks can we successfully demand the huge changes we desire.