Wayland developers had to fix all kinds of things elsewere. The development is not slow. One of the Mir developers attributed the quicker speed of Mir to the work that the Wayland developers did. Further, a lot of stuff in Mir is copied/taken from Wayland. E.g. XWayland -> XMir. They did hire the XWayland developer AFAIC. Wayland developers want to release a good version, apparently for XWayland to work well they need some kernel changes or something. XMir is reported to have issues (check e.g. comments on LWN.net, two mouse pointers, etc)