>They pick Azure because somewhere along the line the mid- and upper-level managers, including director-level IT people, have been made to believe that anything other than Windows is insecure and not suitable for HIPAA.

I'd argue it's less because of a belief that a non-Windows environment is inherently insecure and more that Microsoft very aggressively promotes their products as being compliant with regulations like HIPAA.

This isn't limited to PaaS. For example, I think that Teams will likely gain a foothold in healthcare environments over Slack because MS markets Teams as explicitly HIPAA compliant. (Slack explicitly is not).

Things like that buy them a lot of mindshare when it comes time to choose a PaaS provider.