A few reasons, among which is that the addon needs an account on the Test Pilot site to work. It's not a standalone add-on, so we didn't want to distribute it that way. And we may eventually even migrate away from using an AMO compatible add-on.
That said, we are also contemplating AMO hosting as an interim step. At the same time, we're having a conversation about how alarming those warning messages look. We were also trying this way to kind of dogfood self-hosted addons outside of AMO. So, lots of things converging on this one thing :/
Why is the add-on not hosted on addons.m.o? I thought mozilla thought there was some benefit to users when they only installed things from addons.m.o.