HP threw in the towel on their own cloud a while back, and Microsoft's a logical partner. VMWare's cloudy stuff is too wrapped up with VCE business-wise, and AWS or OpenStack-based clouds aren't "enterprisey."
The Azure perspective on cloud is pretty different from Amazon's, for example. Amazon does let you run virtual machines running your own applications as they were, but is heavily biased toward non-legacy, web-only stuff. Microsoft is taking a different approach in my estimation -- they make all sorts of cool new things available, but seem much more receptive to "hey, come run your crusty old apps the same way you used to while we work to cloudify you." A lot of HP's customers are probably firmly in the crusty old apps camp, and probably want to keep most things in house. After all, why would they be buying name-brand server hardware and storage otherwise?