Microsoft was a lot faster to accept and accompany our privacy terms. The EU data protection laws that you've seen all the fuzz about may be new for the private sector, but in much of the public sector we've actually been under much stricter requirements for years. If you have a big enough agreement with Microsoft, you can even have your data stored on servers you have physical access to and share with no-one else.
AWS was a little slower, but now meet the same requirements, pretty much. Other parts of the public sector is big in AWS instead of Azure.
Google only recently got into any type of real agreements with the EU on data protection, and they're still far from having national agreements. They also don't meet EU requirements on all their products, making it easier to blacklist all Google products than risk your GIS department using Google Maps or your PR department using Google analytics, both illegal for public usage.
For us personally we have more than 25 years of partnership with Microsoft, who have an excellent record on support. By contrast we've never really worked with Google, and we're not typically the ones to try new things. :) Especially not when we're happy with what we have, and since we already have a presence in Azure, I doubt we'll be expanding to another Cloud when pricing is pretty competitive between them. I mean, to run two clouds would also mean our operations crew would need to obtain security certifications for both, which is fairly expensive.
Technically and legally we could sign an agreement for Firebase in particular, and host our mobile apps with a Firebase backend, but as that's not likely to happen, Dart would need to become popular enough to integrate seamlessly and easily with Azure for us to use it.