Why cant bosses have expertise?

It's something we actually talk about quite a bit. We won't do it for Icecrown, but it could happen in the future.

The basic problem is that bosses don't scale with gear. Their health and damage go up. That's it. You might avoid a first tier boss 30% of the time and a final tier boss 60% of the time. That means when that final boss hits you, he needs to hit for twice as much (not even counting that he already needs to hit harder to account for your higher health and armor). If you are a dps class, you might crit a low tier boss 30% of the time and a high tier boss 50% of the time. Why should you be proportionately more powerful against more powerful enemies? If anything it just makes us scale their health and damage to extreme points.

Players generally expect the monsters in an RPG to get more powerful as the characters do. Otherwise the game just gets easier over time. WoW has traditionally not scaled any mob stats beyond damage done and health. I think if we always had done it nobody would think it was weird. You don't get disappointed for example that the bosses hit harder as your health goes up, so why should it be any different of the mobs hit more often as your avoidance goes up? Sunwell Radiance felt odd because it was a sudden, unpredictable addition. If things always had worked that way I think players would just have accepted it.