Probably because they haven't thought of it yet.
If you raise it as an issue, maybe they'll make room for it.
There are so many good reasons to allow pets and, I think an equal number of good reasons not to allow them. We can't really get a pet because we like to travel and we don't want to leave a pet alone or with a sitter all the time. If we ever did get a pet, I would always be looking for pet friendly listings, as I know many people do.
On the other hand, dog and cat dander are serious and common allergies and no amount of cleaning will completely eliminate them so, for the sake of those with allergies, we keep our listings pet free.
Talk about it - tell Airbnb you want it. They have a facebook page. I'm sure they'd be open to adding a button for that.