One of the reasons, and he talks a little about this in the clip below, is that he has never been cut, never knocked out, and gone down maybe 3 times in over 50 fights.
This means his defense is superlative, and I'm pretty sure it's his defense, which is part and parcel of his offense, that has given him this ability to fight professionally for so long.
What I find most odd is that you bring up 'defense' to folks, and many equate it with passivity and somehow lacking in value, even considering defensive fighters somehow lesser than those that charge in full force with little concern for their well being.
I mean I have an idea why this is - Defense is less 'sexy' for the spectacle of ring sports, but it's still strikes me as foolish.
Here are some thoughts about it from a guy who is not really a fan of Hopkins per se, but appreciates the 'genius of defense' : http://www.bestboxingblog.com/?p=1328
Out of interest, the reason he dislikes B-Hop is his view that 'it's only cheating if you get caught' ..... I understand this, but the student of tactics in me can't help but admire what he does and how he gets away with it.
Here a great article pointing out some of his defensive games .... students of the blade arts should notice many similarities ....