Well, even if Couch Potato had a video and became a hit like Fat, SLN, or Amish Paradise, I still don't think he'd be asked to host. It seems strange though that a show that specializes in satire wouldn't think to invite one of the most successful satirists in the past two decades.
Yes, his humor (unfortunately) is seen as "juvenille" by the general public and they don't really appreciate his talent. Most people assume that Al's entire fanbase is children. Children don't usually stay up to watch SNL, so they probably see it as not a good match. However, I do recall an episode starring Maculley Culkin (sp?) back during the time of Home Alone. And they've probably had other entertainers who are usually viewed as "family entertainment" before.
But I don't think Al's "juvenille" or "innocent" humor is the only factor here. For some reason, the general public appreciates humor (even satirical and juvenille) in television and film, but not in music. There hasn't been a mass appeal for comedy music since the 60s.
Scottidog brings up a good point, though, that I forgot about. If Victoria Jackson couldn't get Al on SNL, then I guess Jimmy Falon wouldn't either. Of course, Falon is much more popular on the show than Jackson was, but without a hit video, single, or album to promote (yes, I know his new album is great and all, the best IMO, but by industry standards, it's not a "hit") there's a slim to none chance of Al getting on SNL this year.