Part of the answer is: because they all (and more, from Facebook to kbin) exist. If there would be the one big Twitter alternative where everybody who ever wanted to leave Twitter would go to, then there would be a chance that you’ll find enough interesting accounts to follow there (and then it would become more attractive for others to go there and find enough followers, and so on). But if one goes to place A, one to place B, one to place C, then you likely won’t find the one place where everybody you wanted to follow is.