There is a good link below with some technical rationale (I think the most important part, at least according to Canonical developers, is "Server Allocated Buffers in Mir"), but I'm actually wondering - has Wayland officially added the support for working on the Android graphics stack? There has been at least two proof-of-concept projects, but I don't see either of them being actually committed to the Wayland repository. Given that Canonical is working against the clock it's not that surprising that they are going with their own solution - they obviously believe that they can get it working faster.