First weight. Then resolution. I think SoTA for CRTs peaked at early HD resolution 720. One might be able to modulate the beam faster now. Size. And safety - anyone starting up production now would have to solve the safety issues from scratch. Nothing like a few cubic feet of vacuum imploding to ruin your day.
I used to have a ViewSonic PC CRT that went up to 2048x1536. Sadly my family didn't realize the worth and tossed it out along with a bunch of other CRTs many years ago. In hindsight we should've tested them all and kept the nicest ones.