There's a list of reasons in this article which are all somewhat inaccurate, I just want to address these;
You can only get UWP apps through the Windows store, which puts Microsoft in charge of all data delivery. Microsoft will get a 30% cut of all sales. Sure, that's the same bite Valve takes from games sold on Steam. The difference is that developers can also choose to offer their software elsewhere. They can put their games on GoG, GamersGate, Itch.io, Humble Bundle, or even directly through their website. A UWP app works only within Microsoft's system.
This seems to be a common argument and one that completely misunderstands the philosophy of the UWP / Microsoft Store.
Microsoft have, or are still in the process, of developing app bridges, that allow you to simply port iOS apps to the UWP, or vice-versa. It's easy to port apps developed on the UWP into other formats and back again. There's no exclusivity agreement for games that are published via the UWP / Windows Store method.
Any game developed for the UWP can (or will) be easily be ported to, or from, any other mobile store platform, or a Win32 Code Base via the relevent bridges. You might want to read about the UWP bridges here;
https://dev.windows.com/en-us/bridges/
The philosophy of the UWP is the exact opposite of what you're suggesting here.
It's about making it easier to put your apps / games elsewhere, not harder.
2.UWP games are locked down. So no mods. Which is one of the major benefits of PC gaming.
This is not exactly true. While you can't interfere with the apps files directly, a developer can specify a folder for users to drop files into that the app will then be able to load. As long as the developer supported it, you can make any kind of mod for UWP games just like with Steam games, except for modifying EXE or DLL files.
3.UWP games don't work with capture software. You can't grab footage with Fraps, Bandicam, or streaming clients. In this day of YouTube videos, live streaming, and screenshot sharing, this is unthinkable. It's a deal breaker for anyone who writes about games for a living, which makes me wonder how much Microsoft thought this through.
This is also slightly misleading. Putting aside the fact that Windows 10 comes with DVR capability built in, NVIDIA's Shadowplay will record UWP apps just fine, too. While it's true some screen recording software won't work, it's not fair to say
no software will work. Most of it can work out of the box, or be configured to work just fine.
I'm not defending UWP as a gaming platform, I don't think it's ready either, but these three points I felt I had to address.