The fact that they didn't launch Allo as a replacement for Google Messenger on Android says a lot about the future of this product. It should have been an SMS replacement with full desktop, tablet, and phone integration.
I'd guess there are anti-trust ramifications involved with this decision. And I'm sure carriers and OEMs would have a hissy fit about not being able to make their own messaging apps default on their devices.
It's a huge advantage for Apple since they control the stack and also why, and on top of being this late to the messaging game, Google's messaging solutions, including Allo, will be also-rans.