This is a top ten ranking of former UCLA quarterbacks based on what they did in the NFL.
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. In the NFL, quarterbacks need to be mentally tough and have the ability to make quick decisions under pressure. They also need to be able to handle the scrutiny that comes with being a starting quarterback in the NFL. There were former UCLA quarterbacks like Aikman who thrived under pressure while other former Bruin quarterbacks did not do as well under the intense scrutiny that NFL quarterbacks receive.
Another reason is the skill sets transitioning to the NFL. Successful NFL quarterbacks need to have a wide range of skills in order to succeed. They need to be able to throw the ball accurately, read defenses, and make plays with their feet. There were specific former Bruin quarterbacks who had certain skills that allowed them to play in the NFL for six or more seasons. Other former UCLA quarterbacks, unfortunately, did have the skill sets that made them successful in college and transition to the same success in the NFL.
The fourth reason is scheme fit. Not all college quarterbacks are a good fit for every NFL offense. Some quarterbacks are better suited for spread offenses, while others are better suited for pro-style offenses. Former UCLA quarterbacks played for a coach in the NFL that helped them thrive in the league, while other former Bruin quarterbacks were drafted by a team and coach in the NFL draft and they were not able to be successful in their offensive scheme.
The last reason is to have a legit opportunity to start and play for a good NFL team. Some of the former Bruin quarterbacks got a better opportunity to succeed in the NFL than others. Certain former UCLA players were drafted by a team with a good offensive line and a solid receiving corps.
Of course, there are always exceptions to the above-given reason on why some were successful and others not so much in the NFL as a quarterback. Some quarterbacks who were successful at UCLA struggled in the NFL, while others who were not stars at UCLA went on to have successful NFL careers. However, the factors listed above are some of the most common reasons why successful UCLA quarterbacks were and or are successful or considered to have an underwhelming career in the NFL.