a rather long IRC discussion between the Wayland and a Mir developer(s) on why..
AFAI can tell, a year ago when Canocial/Ubutunu were deciding which way to go, they stopped believing that Wayland would mature into something that fits their grand plans. Instead of discussing these concerns with the wayland people, they decided to fork Android's graphic stack. (and thus be able to talk to all the ARM SoC blob drivers that support it).
So we're going to get one Unity/Mir/Ubuntu stack on everything from Phones to Desktops....
In the end none of their concerns about wayland turned out to be valid. Daniel Stone even has a compositor that does server-side buffer allocation and runs on said ARM SoC h/w. None of the insecure wayland input remarks on Mir's wiki are true, etc.. Not that it matters now, Ubuntu have already invested in Mir and will likely continue to do so; it will after all, being under their control, no doubt fit perfectly into their grand plans.