Hopkins is the only fighter ive ever seen to routinely sucessfully negotiate himself recovery breaks in the middle of rounds.
There is more than one factor, and most of them were already mentioned. Healthy lifestyle, great work ethic, style based on technique rather than on speed or anything else that decays with time, and so on. Also, he was a bit lucky since the division wasn't particularly strong during his reign. It is not to take anything from him, but he simply had no opportunity to fight opponents like Hagler and SRL in MW or Spinks in LHW.
Hopkins was NOT schooled by Roy Jones. It was a boring fight where Jones was running scared. In the 1920's, the fight would have been called a no-contest after the 3rd round because they just were not fighting. I had Roy winning by one point.