Correlation != Causation.
While I am sure it is true that XP has 5x more viruses/infections, it's probably also true that those who are still on XP really have no idea what they are doing on a computer. They're likely older, don't understand computers well, and don't see the need to upgrade from "what works". They're the type to fall for those fake antivirus ads, to not keep up on windows updates (probably don't even know what they are), and may even have an illegitimate copy of windows that prevents them from updating anyway (from their nephew who was being oh so helpful when he saved them money by pirating Windows).
Windows 7 users are likely to be younger, as they want the "latest n greatest", and they're also likely to be more tech-savvy, as they don't mind figuring out something new. Both of those things help when it comes to keeping infections off of a person's computer.
Now maybe none of that is true, but that is the general impression I get from working on various people's computers.
There are always other things to consider when looking at statistics. Just throwing out numbers like this can be deceiving.
And BTW, don't get my wrong, I'm not trying to say that W7 is as insecure as XP - I think it has made leaps and bounds in security - but I just don't believe that the 5 times fewer infections is completely due to these new security provisions.