To be fair, some of it WAS coaching. The offensive coaches were just gawd-awful during the Fisher era and just had no ability to take raw talent and turn them into good NFL players. And coaching across the board was worse than awful in the the Linehan era.
I don't want to give Sam Bradford too much slack for his failings, but he really drew the short end of the stick when it came to coaches during the early part of his career, which is critical for a young QB (most young players of any position, really). Between lousy OL's (which was at least part a function of lousy coaching), injuries, and retrograde offensive coaches, they turned a guy with a lot of arm talent into a David Carr'ed Checkdown Charlie.
There's a reason the Rams were so consistent in hitting on defensive draft picks during the Fisher era while missing so many on the offensive side of the ball. Because we had good defensive coaches, we were able to bring in raw prospects and coach them up into good players. Because we had lousy offensive coaches, the raw players the brought in never had a chance, assuming they would have had a chance in the first place.