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

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..."