Just some quick stuff I was able to dig up:
2003-2007 went to Coastal Carolina University, first year they had a football program. Missed 2006 season with shoulder injury. Other CCU players you might know: WR Jerome Simpson and RB Mike Tolbert
Drafted by Minnesota Vikings in 7th round, waived before season (HC Brad Childress)
2007 Picked up by KC Chiefs, played in one game replacing an injured Damon Huard, tore MCL that week in practice and was put on IR.
2008 Thigpen's final statistics for the season included 21 total touchdowns,
12 interceptions, and 2,994 all-purpose yards. He lost 10 of the 11 games he started at quarterback, six of which were lost by
margins of seven points or less.
2009 Traded to Dolphins after Chad Pennington injury, Henne got one start and then they put in Thigpen for 9 weeks. When Pennington returned, Thigpen was back on the bench.
Basically, he fills in when a guy gets hurt, does ok, but hasn't shown enough to keep the job. Perhaps it was youth and the fact that he was a star in a brand new college program. It may simply be that he needed more time and attention to do well. He showed some skill at the spread offense.