Both are mediocre QBs who had flashes of good play but then regressed. Without the media circus surrounding them, they probably have nice backup careers in the NFL, but with all of the extra hassle it's just not worth it to teams to deal with.
Look around for any other backup QB who is in any way controversial... theyre just not worth it.
You can argue that Kaep isn't a backup QB, but his play recently has not been good. He got beat out by Gabbert for heaven's sake, and then came in and didn't do much better.