[fryguy]

I think this is mostly to do with cookies not working cross domains (for obvious reasons).

[jessaustin]

This seems likely. Still, with a bit of planning they could have gotten around this. Everything dealing with identity could have been served from a single domain. There probably would still have been issues, but they would have been resolvable.

[TazeTSchnitzel]

But Google manages to do this, I'm unconvinced.