One, perhaps more minor reason is because the replies you get will more than likely consist of "choice A" or "choice B" sometimes with a small jokey statement to acompany the choice.
The second, more major reason, to me at least, is that they could easily devolve into flame wars. For example, someone makes a VS thread entitled "PS3 vs Xbox 360: Which is better?" and you can easily see how that is an opening for a flame war. It will lead to die hard fanboys of either console attacking the die hard fanboys of the other with (more likely than not) homophobic insults and racial attacks. To avoid that, we tend to avoid VS threads altogether here.