Email is often a good analogy for questions like these: Why do you pick one email service over the other? Maybe you think the domain is cool. Maybe you heard that they are reliable or have features that others don't. Maybe your university, company or other community you belong to runs its own service. Or you decide to host your own. Or you really have no preference and you just go with the one your friend uses.
We just don't have just don't have the gmail-equivalent catch-all choices yet. Maybe that will come later or maybe it won't.