Because the world would have been a *better* place without fvwm, gwm, KWin, compiz, XMonad, Awesome and dwm! Yeah, we can come up with just one perfect window manager which has all the properties of all those projects no matter how diametrically opposed, and twm would clearly have been the right place to start! The first window manager anyone thought of should just have been tied into X at the start, and then everyone could extend it as long as they didn't mind hacking their X server!
(This sort of diametric opposition, lack of agreement, and lack of clear right answers is entirely *why* X went in for 'mechanism, not policy' in the first place. I'm astonished that this has apparently been forgotten by the Wayland devs, given that some of them are the same people who came up with that policy for X. What has changed to make it a bad idea? Nothing, as far as I can tell, except for a desire for consistency whether or not people like the consistent result more than what they could have had otherwise.)