It's mostly a lack of familiarity with the system and lack of experience (including bad) situation. Better the jag you know who was with you in camp or ran a similar system for someone else than some guy who just got cut (from a team that runs a scheme unlike most everyone else who has never started).
I decided a long time ago there are probably as many potentially viable QB's who come out of college never to get a shot as there are QB's who make it. Mostly because coaches and coordinators become enamored of a handful of guys every season and by the time most of them bust guys who might have actually developed are selling insurance or working for investment banks. Then the busts get a second shot as a backup based on they started once and might be misunderstood...
Compared to other positions where everyone needs a couple of guys only 3-5 QB's that get drafted every year ever get anything approaching a clear shot at hanging around let alone starting in the NFL. They should really allow teams to carry a developmental QB on the PS over and above the 8 player limit. I know it was talked about a while ago, I think when NFLE went belly up, because of the dearth of quality starters being developed due to roster limitations.
