If you actually looked a little deeper than the "23 games, 12 wins", you might see that Trentative started out on fire and has gone cold during his string. Some of these other guys that you're comparing are the exact opposite. Now why?
I'd say Trentative's early success was precisely because he was the anti-Losman QB. He, like Holcomb previously, came in and had quick success because he got the ball out very quickly and got the offense into a better tempo than JP ever could. Instead of slinging it deep to Evans, he was eating up the short stuff that the defenses, which hadn't seen him before, were giving him. Once the book was established on Trentative and defenses started to plug up the short passing game, he started to struggle. Not coincidentally, he has struggled in the red zone.
With the other QBs, I'm going to say that they followed a great QB's natural maturation process. In college, they were used to making any throw they wanted. In the NFL, they had to learn exactly when and where they can and cannot make a throw. They have to get used to a new offense, new teammates, speed of the NFL game, etc. It takes some time. But once guys like Peyton Manning got through their growing pains, their raw athletic ability, maturation, and confidence combined to create a QB that would consistently perform at a high level.
There are far many more examples of QBs that fail. And, unless Edwards can develop a deep game and figure out how to effectively operate an NFL offense in the red zone and in the face of pressure, he will be joining that longer list. This season is riding on his shoulders.