Because the people who developed X11 (that is Xorg) haven declared that. Maybe they should have named it X12, but they didn't for whatever reason. However the people doing the work have already given up on working on X11 they gave up on X11 beyond the bare minimum almost 10 years ago because some real issues with X11 as a protocol are not fixable.
There were other attempts to a successor to X11, but they never got the support of people doing the work on X11 (in part because they didn't understand the problem with X11 and so kept many bad things while 'fixing' things that were not broken)
Which is to say: you have two choices: get involved with continuing X11 development, or jump to Wayland. Throw a couple million $$$ per year at X11 (either pay developers, or convince a dozen developers to maintain X11) and I'll retract my statement, until then X11 is dead. If you cannot do that then Wayland is your only option.