Corporate inertia. Sibling comment uses the term "hostage situation" which I admit is pretty apt.

Microsoft is an approved vendor in every large enterprise. That they have been approved for desktop productivity, Sharepoint, email and on-prem systems does not enter the picture. That would be too nuanced.

Dealing with a Large Enterprise[tm] is an exercise in frustration. A particular client had to be deployed to Azure because their estimate was that getting a new cloud vendor approved for production deployments would be a gargantuan 18-to-24 month org-wide and politically fraught process.

If you are a large corp and have to move workloads to the cloud (because let's be honest: maintaining your own data centres and hardware procurement pipelines is a serious drag) then you go with whatever vendor your organisation has approved. And if the only pre-approved vendor with a cloud offering is Microsoft, you use Azure.