Most of the time it's just part of the bundle. If you are heavy into SQL Server, Office 365 and Power BI then there is a BIG chance you are going to use Azure for whatever the reason.
People who take Azure up without previous MS product experience...not sure about those.
There's a few, mostly retailers who don't want to give money to Amazon as a direct competitor, for them Microsoft/Azure is more of a neutral party, and most businesses already use Microsoft in at least some fashion so already have staff internally familiar with MS products (as opposed to say, going to GCP instead).
For everyone else, it's like you said. "Eh, we are already knee deep in the Microsoft stack, why would we pick anything else?"