There are two possibilities: Either it’s really the best choice among the available frameworks (very questionable), or they picked it regardless. Both reflect badly on Microsoft, given what React Native is, and given how central the Start menu is to the Windows experience.
Here's one: Microsoft management heavily incentivizes their developers to use LLMs for virtually everything (to the "do it or you're fired" level) and the LLM (due to its training data or whatever) is far more able to pump out code with React Native than their own frameworks. This makes it the right choice for them. Not for the user, but you can't have everything.
I don't have any inside information; I'm running with the hypothetical.
If you know why it was chosen and if it was a bad choice compared to other frameworks, please do tell.