Most corporations have Microsoft already greenlisted as a vendor.

Making it possible to buy something from Anthropic might require tedious paperwork for many of them.

you can also get a service contract via MS quite easily/cheaply, which mightnot help you with hard problems but does solve the easy ones. example: in earlydays we bought OpenAI API directly and via Azure; when we needed account service we got it immediately from MS instead of waitlists from OpenAI.