When you're dealing with Google, Microsoft, Amazon, etc etc.... someone's going to be slow, someone's going to be fast. Maybe Azure paid for first dibs, I don't know. From experience I'd say probably Azure just was fastest to say "ok". The mechanics behind delivering new images is pretty minimal, especially compared to the lawyers and politics.

disclaimer: I work for Canonical, but not on anything at all related to this, though I do really love the sound of it.