There's only a few reasons we and our clients use SQL Server. It works, it's mostly cost effective, it can be installed on premise and no one has gotten fired yet for suggesting it, (aka name recognition/trust). (We will ignore interoperability since currently the applications we write are the main way our clients access the data.)
Now here's the problem for Redmond. Other solutions work, are cheaper and don't require/pimp the cloud. So name recognition/trust/lockin is the only thing keeping competitors at bay. If you don't require the "whole stack" of SQL Server, you might not need it at all. Also, if I can provide a service that doesn't require any MS software to my clients via a private/semi-private cloud or an on-premise service, what's Microsoft to do? There's more to OSS than Hadoop and PostgreSQL. Through a browser, no one knows you are a dog, or necessary what software the server runs.