Who knows why, but they already are. Apparently hotmail in the early days used Linux for some servers. Linux is pretty common in Azure.

Hotmail was built on top of Unix by someone else, and then Microsoft acquired it.

Yes, Microsoft does do Linux these days, they even have their distro, but this still does not answer the question of why they would replace real Windows with Wine and risk compatibility issues, if they don’t have to pay anything for licenses.