Because Adult sites will hear what they don't want to hear.
I had this discussion with a client years back. They created a poll like:
What do you think of our members area?
Too Slow
Not Enough Content
Too Hard to Navigate
Can't see menus, I'm not using IE
I asked them where they came up with the options, they said, well, they're problems on the site. If a site owner knows there are problems, why do they need to ask the surfer?
I think a poll like that predisposes the surfer to the answers that the sponsor doesn't want to hear.
I would much rather have a textarea box that asked for a surfers comments.
If you're talking about polls and voting on whether a picture set or video is hot, I've seen a number of attempts at it, but, surfers generally don't tend to take the time that they used to.
However, I do have one site where the average visit time is 251 seconds.
So, there are things that get surfers involved.