Basic ecology, man. Monocultures suffer from parasite plagues more than endemites. Popularity of Windows and the fact that there is just a handful of different configurations make them an easy and profitable target. How many boxes are out there with that precise version of the kernel that you are running? Of the software that you are running?
Also, in Ubuntu, you have security updates on an almost daily basis, rather than waiting for the next service pack.
Furthermore, 99% of the software you are running comes from trusted sources.
That said, AV might still be useful, for example to check Windows files, including your WINE programs. Also, in fact, there are some Linux viruses out there; but they have a very hard time spreading and so are not, in general, a major concern. Read more here.
And, of course, Linux does not protect you from bugs in Javascript or web browsers.
You can see a brief description of Linux antivirus programs here.