Ubuntu is Microsoft's "favorite" distro, they do seem rather made for each other.
The Azure dev tools on Linux i have tried are all Snap only, or a tar file and maybe an rpm/deb. At the moment they seem to avoid Flatpak like the plague.
I would argue Microsoft's choice of Ubuntu is one based upon practicality and social inertia.
As far as I'm aware, Ubuntu is still the biggest player in the consumer Linux market not counting Android. Given the need to balance upkeep costs with range of support, I can see why Microsoft chose just Ubuntu and threw the rest to the wind.
It's pretty on-brand for Microsoft to select a consumer Linux distribution when building out their B2B cloud product, instead of (for instance) one of the leading business-oriented distributions.
Azure StorageExplorer is one i can recall of the top of my head. Although it's true a lot of dev tools will be CLI and not have too many dependencies (you would hope).
Flatpak is essentially RedHat/IBM tech. Why should Canonical even think about supporting their competitors. Just because multi billion corps like RedHat/IBM have the audacity to ask for donations to develop their products doesn't mean Flatpaks are in any way neutral territory.
Flatpak is supported by Endless [1], SUSE [2], Codethink [3], Collabora [4], Igalia [5] ... none of these businesses have an issue with "supporting their competitors" because that's an inherent part of doing open source as a business.
Smells like that old MS attitude to me.