There is not a lot of incentive for a developer to make a UWP app. Mobile was the place where UWP was predominant and targeted. While Xbox, HoloLens, PC can use UWP, there is no demand for them over there.
Most people do not fire up their Xbox to check Twitter, look at their bank account, or message a friend. They play games or watch movies. Aside from Netflix, ESPN, and the like, there is no reason to create an app for Xbox.
HoloLens is not a consumer item. They cost $3k and are not released for consumption yet. On top of that, even Microsoft is saying they are not for consumers. Apps for that will be targeted for specific industries and not be consumer oriented.
Most people do not use apps on their desktop or laptop. They use W32 programs. The majority of people who use them own a Surface. That is still a small segment of the PC population, and always will be.
Developing apps is expensive and time consuming. Right now, there is very little demand for them in the Windows ecosystem. If you talk to your friends and family, they will probably tell you that the only apps they use are on their phone, and if they were to do the same task on their computer, they would use the web or a W32 program to accomplish that task.
Microsoft is looking to create a new product category to drive app development in Windows, but until then, UWP will smolder instead of set the world on fire.