like it was mentioned before i_fallenhero, there are simply less windows linux/mac, and therefore less damage, and therefore less incentive to program evil software for those operating systems. but that doesn't mean the incentive is completely gone.
lets say you have 3 banks.
two tiny banks, holding 5% of the world's money each.
another big bank, holding 90% of the world's money.
if you were going to rob a bank, which one would you rob?
The smallest. The bank with 90% of the money is going to have much more security, so the 5% bank would be an easier target, and it's still 5% of the worlds money... even with a fraction of that , I could retire.
But your right, a lot of it has to do with how much each is used, though I'm still convinced that Linux is less susceptible for a couple of other reasons as well:
1.) Installation. Compare the installation of windows vs Linux. I personally use Gentoo/slackware, and the gap in required knowledge from installing windows to installing Gentoo/slackware is tremendous. A trained monkey can install windows on a computer, but most Linux distros require a little more know-how. What this means is the average Linux user is going to be far more tech savvy than the average windows/mac user, and they're going to know when they should/shouldn't trust something far more often than a windows user.
2.) The differences in ease of running/installing a program. To install something in windows, doesn't take more than one or two clicks. *Almost* all the programs in linux (that I've dealt with, on gentoo/slackware again) require MUCH more thought, and often a glance at the readme file for instructions on installations/requirements. All of which is again, going to make the linux user more likely to move on. (obviously only applies to malware that is intentionally installed, and yes, this happens all the time to windows ppl)
3.) mattman059 makes an excelent point, and the fact that the distros are open source and therefore have a much larger base working on improving them, and aren't as tied to release dates the way the big corporations are probably has a much better impact on security. (That and the fact that every nit-picker in the world has access to the code and can help debug it.)
A lot of it has to do with who is using it imo, but then I think every software problem in windows is user error when you get right down to it.