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.
