Or because you already use Microsoft mail, so you already have Azure AD there, and they sent you some credits in an email, and then...

I think Azure is cheaper for certain workloads as well, and at one point had DCs in places where AWS didn't. But it's mostly the "we already buy X Microsoft product, and they cost about the same, so..."

Quite a few businesses are still simply more comfortable with any idea if Microsoft does it than otherwise.

From what I’ve seen up until now it’s often the last option. I’ve seen comparisons between the major cloud providers, but those have often been on the level of “I can boot a VM with x cpu on provider y and z”. With these kind of comparisons a major deciding factor then is discounts. And that’s an area Microsoft excels at.