Easy - make it less viable for distribution than UWP. Add some little inconveniences to win32 that won’t be a roadblock, but would be enough to make developer think more about UWP. continuously release cool features for UWP only, create new inconveniences for win32 and consistently market UWP(Much more secure! Big exposure for your apps!).
Why do they should care about promoting/improving win32 if UWP earn money for them? If they wanted to earn money with selling W10 they would take another route for its promotion, but they clearly just want to maximize userbase in order to maximize possible profit from UWP. This is not bad, as a platform owner and developer it’s pretty smart thing to do. Also, as a platform owner, you will see Steam and other stores as rivals - they distribute same content as you, however they don’t pay a penny to you for using your platform.
A logical step for them to either mark other Stores as bad/insecure or demand a cut. Is it bad? No, I guess! It’s Microsoft platform and their rules, other companies did the same!
The problem - everyone except Microsoft lose in this scenario.