There are many reasons why certain UCLA quarterbacks are and were successful or a not in the NFL. One reason is looking at physical attributes. Certain NFL quarterbacks need to be physically gifted in order to succeed. They need to have a strong arm, be able to scramble and be durable enough to withstand the punishment of the NFL game. Some former Bruin quarterbacks physical attributes helped them be successful in the NFL, while others were limited by their physical characteristics and were not as successful in the league.
The second reason is the mental makeup of being able to thrive under pressure and overcome mistakes and bounce back in games.