The sad thing is that this is the most popular Linux WM.
Not for long.
Shuttleworth wants to ditch both X and Wayland and use a brand-new display server with blackcjack and hookers.
You should read this as: if you want to use unity with X, you're fucked, because they don't give a shit for X. And if you want to use their new server once it comes out, you're still fucked because anything not written by Canonical will likely not work on it, and half of things that are written by Canonical likely won't either.
Ubuntu isn't that popular anymore. In fact, if you pay attention to
DistroWatch, the bastard child of the spacefarer from south africa is on the decline, with Mint having taken the top spot quite a while ago.
Now: guess why.