Probably for similar reasons people would rather change leadership at, say, Mozilla than fork Firefox. Again. To create another fork nobody cares about. Again.
Politics is, often, about centralization and coordination of power. It's a lot more effective to change leadership at an organization with good ideas but questionable people than to split power and focus by forming a competing organization. The two resulting organizations may end up politically weaker than one organization (especially if they can't coordinate their efforts because the membership of one of them expects their org to boycott the other org for the reasons they split in the first place).
Jill Stein may, for example, have ideological purity over the Democrats but she'll never be President.
In any case, I hear the FSFE decided to split from working with the FSF when the FSF re-instated Stallman. I'd prefer to have an org with more direct influence over US law and policy, but I'll happily support FSFE since it's the closest thing I have to supporting free software as a concept without supporting continued discussion of age of consent on the side.