I think going down in weight was the big problem. You will rarely ever find a boxer that goes up in weight by 20lbs and then trying dropping weight for the next fight. He should have stayed at heavyweight. Against Ruiz he still had his speed and was doing fine, then he dropped down back to lightheavy weight, big mistake.
As some poster's mentioned, Roy's style relied on reflexes, confidence and the ability to react naturally without thinking, which are basically reflexes. Once the reflexes and confidence took a hit, he was already 35, and it was too big of a setback at that point in his career... He never got back to the elite level. Not to mention, his punch resistance really seemed to decline around this time, which didn't help.