It's this. Microsoft doesn't care what model they use, they sell an integrated office + AI product, model agnostic. They'll offer whatever models they need to continue to sell their product globally.
As much as Copilot sucks, I think Microsoft has the right idea. Model is a dumb pipe, basically an ISP. The actual value is what's built over top of it, and packaged up nicely for an enterprise to buy off the shelf and be immediately useful. I imagine a future where picking an LLM model is no different from choosing who my internet provider at home is. They all do the same thing with varying levels of reliability, customer service, speed, etc.
China was behind the frontier before they suddenly weren't (K3). Just using American models would never have achieved that (not that they could have anyway, but the point still stands).
If they want European models (whether for regulatory or other reasons) then they have to accept it, whether they like it or not, because Mistral is not competitive. Personally I don't believe Europe as a whole can compete, being cutthroat these days is just not in their culture anymore like it is in Silicon Valley in the US and China as a whole.