Because they voted him in. With a record number of new memberships in the wake of the GE. And in a climate where Brexit was in part a (misguided) democratic revolt against the political elites and their perceived rejection of the average voter's views, I don't see how the Labour Party seeming to do the exact same thing, to one of the few sections of society that still largely support them, is going to suddenly ring in a new dawn of cohesive harmony. They never forgave Clegg for his "betrayal"
However much you/I/we may personally agree with ousting Corbyn, ideologically (or even tactically) the timing and manner of it looks terrible. Especially as this should be the perfect time to be grilling the backtracking bullshit of Gove, BJ & Farage, rather than creating another, self made, internal crisis, and ESPECIALLY (all caps) as there's no real stand out alternative to ring in this supposed new, strong, Labour dawn. Again, omni-shambles.