You know there are many other factors to consider when signing a backup QB?
How comfortable they are being #2 behind an entrenched starter, what they offer in the film room and veteran experience, as well as mentor ship.
Now I don't know if Kaep is deficient or not in any of those areas. But I'd imagine he wants to start and won't be content as a backup, so half the league won't want him based on that. His experience game planning and executing different schemes and systems is also probably less profound than let's say Cassel, a guy who has bounced around and practiced and played in many different systems and had many different coaches.
Backups aren't just QB's 33-64.