1) The actresses that appeared nude would have to sign nudity release forms. These women might not be too keen on having their naked bodies saved for posterity on a cable comedy show.
2) The actresses that appeared nude might not be nude at all, in fact in many cases when a TV show films nude scenes knowing that they are going to be blurred, they will put tape over the private parts.
If you saw season 1 volume 2, there WAS an instance of unblurred nudity; There was a sketch where a porn star giving the fox a christmas pressent; a nude photo of herself. This photo was unblurred on the DVD. This is because 1) the woman who appeared nude gave permission for the photo to be shown on DVD, and 2) the woman was actually nude under the blurs.
Also, the DVDs have been uncensored in the language department, all cussing is unbleeped.
Basically what it comes down to is that these women aren't whores. They are actresses trying to carve out a living. Yes, they are using their bodies, but that doesn't mean they want sign their naked bodies away to be your DVD masturbation material.