I think the coaches lost confidence in him and once they did, he wasn’t given the opportunity to overcome his slow start. The Dolphins OL and its inability to run block was also a big issue when he was with the Dolphins.Hell... I really liked the Edmonds move, and I still can't figure out why he was so bad here.
Look at his stats from Denver... they are pretty good. He averaged 4.8 yards per carry there.
When he was here, it almost looked like he ran to the wrong spot on every play, on purpose.
The only thing I can figure is that he just didn't 'get' the offense, because he isn't a bad player, even though he was here.
As you point out, he was very effective with the Broncos and he was also an effective RB for the Cardinals prior to signing with the Dolphins. So it appears that the issue was more the Dolphins OL and the offensive system than it was Edmonds.