If I were to guess, to avoid having to go through the meetings and internal bureaucracy for it. It'd literally be cheaper (after counting in person-hours of time) to just pay for an external vendor.
There's no extra meetings or internal bureaucracy for using Azure internally. It's painless to get an internal subscription for Azure that can be used by your team.
Perhaps that was not the case in 2013?
Did you try using Azure to run Linux workloads "at scale" (100+ nodes) in 2013?