He gained 20 pounds between the Rose Bowl and the Combine (when you'd think he'd be working maniacally to prove himself). He missed his team interview with the Colts during the lead-up to the draft. He left the rookie symposium several days early, and then filed a grievance when he got a $10K fine for not finishing it. He had a college reputation as a party animal and his work ethic was already questioned before he was selected. And he had no interest in any of the strength and conditioning or O.T.A.'s in his rookie and 2nd years with the Chargers.
He was just lazy and wanted to party, and once he had the $11MM signing bonus, his only motivation was not to be humiliated (which he couldn't seem to avoid on or off the field). I agree with you that the Colts had better offensive players, but Leaf would've been dreadful no matter what.