The XWayland support wasn't ready (nor even fully proposed for pulling) in time for the X.Org Server 1.15 merge window that closed this week. XWayland is what allows legacy X11 applications to run atop Wayland by using a root-less server.
Part of XWayland was proposed for mainlining, namely the code changes that cause ABI breaks for the video drivers. However, even that wasn't merged for the X.Org Server 1.15 release.
For those curious about XWayland OpenGL performance for gaming, I have XWayland benchmarks as of this week.
Besides XWayland, the XMir compatibility layer wasn't merged either but it also hasn't been proposed for mainline by any Canonical developers.
The earliest we'll now see mainline XMir or XWayland support will be H2'2014 with X.Org Server 1.16. Distribution vendors, however, can always patch their xorg-server packages if they wish to ship the X11 compatibility support for these next-generation display servers.