The problem with UWP is that nobody liked or wanted it, so obviously it died. Developers got locked into a walled garden, Xbox users saw no benefit (until recently with emulators, but I digress), Windows phone users practically didn't exist to build apps for, and PC gamers got the real short end of the stick with everything becoming a buggy, locked-down, performance-hampering, feature-lacking mess.

Outside of letting Microsoft half-ass port some Xbox titles to PC, it was pretty much useless.