I think one of the main issues that many lifelong Labour voters have now is that they cannot imagine a Labour Party in its current state, where most Labour MPs have very little confidence in thr Party leader, as being able to form an effective government.
This problem is so huge that, even with a decent manifesto, people don't see the point of voting Labour in this general election, because it could never be implemented.
Meanwhile, for all her faults, Theresa May does seem to have inspired in voters, even ones who wouldn't normally vote Conservative, a confidence that she will get things done that they want done.