I think they are choosing react native because they genuinely like react.
I don’t remember which video it was exactly, but one of the higher ups talked about future technologies within Microsoft and how they were doing more and more GUI-based thing with react. And if you look at it, they’ve done pretty amazing things within the JavaScript eco-system in general. Office 365 is amazing, Visual Studio Code is amazing and it just wouldn’t make sense for them to go from typescript to dart.
Especially when you consider how unfinished flutter is. We’re a C#/power shell with a little Python shop with a lot of Enterprise Microsoft techs. We still considered Flutter because Xamarin wasn’t working out for us and we’re not big on JS or big enough to do native, but flutter just doesn’t fill our needs either. That’s anecdotal, but the difference is that react and react native are proven techs and flutter still isn’t.
Yeah Flutter's main focus is mobile, they are working on Desktop Flutter, you can try it out on the master channel.
It looks like they are making progress, Chris Sells, a Flutter PM, tweeted
"While things have been delayed on the desktop side for Flutter due to the current crisis, the team has been working hard to bring both Windows and Linux support to alpha. I think you're going to be happy about what you see."
https://twitter.com/csells/status/1261036199294062592
Maybe befause React native uses OS UI components while Flutter creates everything from scratch. https://microsoft.github.io/react-native-windows/