Why UWP? Who buys from Windows Store?
Not my parents. They started computers with buying software in boxes, but in the modern age almost all of their digital purchases are done on smartphones. Neither of them have any idea what Windows Store is. One of them uses Steam for games.
Not most power users I know, most of which bawk at UWP apps and the concept of a walled garden store on their desktop computer. Many power users are more likely to use Steam to buy apps despite it being gaming focused.
Anecdotally I see basically no group that would prefer Windows Store except out of ignorance or necessity. Buying on Windows Store means you don't get the Mac version if one exists. Admittedly, the same caveat applies in reverse to the more successful Mac App Store, but it's still something people have to consider.
Microsoft has been pushing Windows Store pretty hard for a long time. They came to my University back when I was in college, a bit after the Windows 8 launch. They talked numbers but only in terms of the whole Windows install base and not even Windows 8 adoption. Definitely no numbers on Windows Store.
To this day, I don't know if we've seen good stats on how much apps that were built and sold exclusively on Windows Store have fared. No doubt games have sold well - Microsoft can use its Xbox brand to deliver exclusives to Windows Store and not Steam - but I sincerely and absolutely doubt that success will translate to app developers.