Canonical have an official cloud partnership scheme, so why is Ubuntu doing an exclusive launch-day cloud image deal with Microsoft Azure and not their other cloud partners?
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.
Images on other services are coming soon. I guess Azure guys just made it happen sooner; either way, I'd rather have one cloud service to test today than none.