I suspect that that most successful QB transfers are the result of coaching changes. Maybe a new coach brings in a new system that is less of a fit or they bring in their own QB. Possibly they follow their coach. Most QBs not getting playing time will simply find the competition just as difficult but adding a restart with everything new. The exception is the very top programs, like Alabama or Texas, can bring in multiple QBs that will be capable of becoming a top 20 QB. In this case, one will fall off and be driven to success elsewhere. Their success excites desperate teams that then begin to chase an upgrade only to find that other than this exception they are chasing unicorns that only pop up occasionally like Love at USU.