With that being said, the typical appeal of Azure (and likely other cloud providers) are the PaaS services. Those typically do have Windows VMs sitting behind them, although we are starting to make Linux an option on most of those.
Very few people start a project using SQL Server if they're not otherwise using MS stuff.
Sometimes it's because a long time ago the service was done by a whole different set of people and now some other group has inherited it and just doesn't want to port it to something else even though everything else they use is open source. These are typically the groups who go to the Azure SQL PaaS service because they don't want to manage something that they don't really care for.
There is a ton of fragmentation out there :)
Postgres is free and usually good enough. If you are going to pay, Oracle is generally better. The ground that mssql wins for a non-microsoft shop is verrrrry small.