It's not some sort of anti-Celtics bias. That's just stupid talk and is entirely refuted by Hawyard going up in the rankings.
It's simple folks. Your ranking tends to drop when you only play on one half of the court.
Lowry put up more rebounds, assists, steals, a better shooting percentage and was far better than Irving on defense. Conley put up similar offensive statistics except for points (5ppg fewer) but is so far ahead of Irving on defense that he gets the nod and rightfully so. When you factor in that defense is half the game it's really hard to justify putting Irving ahead of either guy.
That's the bad news. The good news is that it's far more likely that Irving can improve on defense than Lowry/Conley will improve on offense. So we all hope next years' rankings reflect Irving actually trying to contribute something on the defensive end of the court.