I am going to assume that this is quite self-explanatory, but almost all movies you watch are often shot in 21:9, yet every single TV I find are all 16:9. This baffles me.
Probably because most people wouldn't care about 21:9, and a lot of content is just going to end up cropped.
Anyone who really cares about 21:9 probably has a projector.
I would speculate that the ultrawide monitor market is a toe in the water; testing for consumer interest.
Really, though, content delivery isn't primed for that. Most films are not anamorphic, but many are. The BD standard is still playing catch up, now that 4k TVs are becoming more affordable. Anything delivered to a home at the 21:9 ratio is just going to be letterboxed 1080p, so a TV that's made for that would be pointless, at this point, as every 16:9 and 4:3 program is going to look terrible.
Here's a Wikipedia link on the format, itself. Note that it requires specific equipment for shooting it, meaning that it's a stylistic choice for each film that uses it, rather than a matter of ubiquity. Most content is 16:9, these days.
From my understanding the reason is simply that Movies are 21:9, however TV shows aren't, and are normally shot in 16:9 or in earlier formats (4:3), so if the tv was 21:9 you would always see a black bar on the side, everytime you watched a tv program (rather than just the black bars at the bottom when watching a film).