You've clearly seen the type of offense he likes to run. Now compare it to a Chargers team that lacked running backs. They tried to make Eckler their every down RB and he's simply not cut out for that.
He's run a top ten offense 4 out of his 6 years as an OC. One of the two bad years he had Dalton, Gilbert and some guy named DiNucci starting 11 games for him, the other was the RB-less Chargers. But sure go off just that one year with the Chargers.