I think the question from their perspective is, why would they? What's the market? What hole does it fill in their offerings.
There's a big part of the answer. Another big part is "...and what gun are we going to put it in?"
A revolver isn't usually terribly happy with bottlenecked cartridges. An auto, once engineered to feed a certain cartridge design (like a stubby, straight-walled 9mm, .40, or .45ACP) doesn't usually lend itself to being easily tweaked to feed a radically different cartridge design. Getting a 1911 to feed 9mm is a tough trick that not many builders or manufacturers have managed to do with anything close to perfect success. Trying to make one feed a tiny, pointy, bottlenecked round would be maddening.
So the manufacturer pretty much has to build a gun from the ground up to handle the oddball round. That costs a lot of money. There has to be a pretty solid expectation of a return on that investment. FN expects (or did expect) to make that up through companion sales to go along with their P90, and a few through the sheer novelty of it. I imagine they're making their money back, but not by the barrel full in such a way as to encourage other manufacturers to invest in their own dedicated platform. Waiting for S&W, Glock, Springfield, Colt (uh...HA!) or someone like that to jump on the bandwagon when there's really no niche for this in the civilian world is probably going to go nowhere.
Maybe it's better for self-defense than a .22 (even with the neutered civilian ammo) if your grandmother really can't handle the recoil of a more effective round, and maybe it is fun to plink small game with for guys who don't mind paying for very expensive ammo, but those are pretty small market pockets to base a complete new design and manufacturing facility around.