Being the most popular does not mean the best choice, especially in Amazon's cloud where most people would be using it for development and testing, not necessarily production. The last few places I worked production was all RHEL. Development and testing projects went to EC2 and CentOS. This was not a "CentOS is better" consideration, it was exclusively a pricing consideration. Ubuntu is the same, where it's mostly free and lots of the fad followers still think Ubuntu is better than other OSes because it's simple to setup. For a workstation I'd agree that it's easier for a non Admin to setup. There is no advantages and some disadvantages when using it for a server other than a simple Web/DB server.
IMHO the problem with any of these statistics reports is that it does not demonstrate reality in any way, shape, or form. Like all statistics, it's intentionally worded to mislead people. From the title, you would think that the Hyper-visor is Ubuntu but it's not. TFA also makes a wild ass guess because Amazon said it's the most used for them and they own 57% of the cloud market. You don't have to be a math wizard to see how that speculation could easily be wrong (Amazon never said that 98% of their client nodes are running Ubuntu).
Personally, I see Ubuntu exactly like MS. It's controlled by the Brits who have more intrusion ability by the Government than the US (with US help of course). I don't trust either, and won't use either. That does not mean I'm running out to pay for RHEL licenses. I'll use a good trusted free OS like Debian or CentOS over MS or Canonical's Ubuntu. Sometimes free makes lots of sense, and other times you want the pay for support.