There’s multiple dimensions to what’s going on inside the Labour party and outside. Then with Momentum as an extra layer on top of that.
So the economic one. Lots of people are kinda sick of neo-Liberalism in its many forms. From the ex mining towns that have been fucked to young professionals in London who will never afford a decent house. So people are sick of austerity, are fine with railways going public again (the Manifesto only really committed to assessing it as franchises came up or setting up a Nationalised operator to compete for them, like in Holland), or more NHS spending etc. The more out there stuff like 10% shares going to workers wasn’t in the manifesto, though it was said out loud. So it’s a broadly social democrat manifesto, which Momentum, MPs and the leadership are all behind. The ones who are true died in the wool neo-liberals have fucked off to the Lib Dems.
Also everyone being sick of the current economic model, plus the Lib Dems being in government for austerity (the quisling fucks) is why the progressive side of voters still haven’t ditched Labour for the Lib Dems yet.
Then there’s Brexit. Where Corbyn more particular comes in as everyone thinks he’s not very Remainy and had to be dragged kicking and screaming around to Remain. But it’s also an indicator that Momentum isn’t just his tool as they were some of the people forcing the Leadership’s hand. Weirdly a big campaigning group of young people, who have no knowledge of old school Socialism all want to remain. Momentum also differ from Corbyn in being way ahead of him on the Environment or Green New Deal kinds of policy, and way more pro immigration. As again young people.
So Corbyn’s problems are more that nobody trusts him. His stance on anti-semitism has been evasive and dodgy. The PLP basically just don’t trust him that much but know they can’t move against him. Then he interviews badly. Can’t score points off of Johnson that easily somehow. Plus a hostile press etc.
So the MPs haven’t jumped because they do actually mostly agree with the 2017 manifesto, as does the membership. Just trust in the leadership is really bad. But most those MPs have been members their whole life and it seems unthinkable to them to leave. Then they prefer that policy platform to anything the Lib Dems will put out, so where they gonna go? The Greens? They’re just trying to wait Corbyn out really.