Why does a Demon Hunter take more damage than a Monk with near-identical stats?

Monks and Barbarians take 30% less damage than the other 3 classes. This is an inherent attribute valid from level 1.

Now, 30% is significant but I'm not sure it's enough to justify the radical difference you're describing. How certain are you that it's not just your general perception? Because in practice, a Monk does not tend to just stand there tanking. He tends to hit back, which often causes a "hit" animation on monsters which prevents them from attacking. This is before, of course, we consider that a typical Monk is loaded to the brim with skills that stun enemies, blind enemies, decrease enemy armor, increase the Monk's armor, etc.

There are a couple of reasons for that.

Monks and barbarians take 30% less damage by default. Some other reasons include much higher armor due to the mote used skills/passives for most monks Some of them are:

• hard target rune for mantra of evasion boosts armor by 20%
• mantra of evasion pops a 2 min 15% boost to dodge, with an additional 15% for the 1st 3 sec
• Intimidate rune for mantra of conviction decreases all damage mobs do by 10%

• time of need rune for mantra of healing boosts all resistances by 20%

• One with everything passive sets all resistances to equal your highest resistance. Due to this passive and above stated time of need, monks usualy have very hihg resistance values (well over 1000, which brings them into the 80%+ dmg reduction range

• guardians path pops a 15% dodge chance if dual-wielding
• resolve passive reduces all damage from mobs you damage by 25% for 2.5 sec

• sieze the initiastive makes your dex also increase armor the same way strenght does

• cripling wave primary with concussion rune reduces all damage from mobs you hit by 20% for 3 sec

• fist of thunder with lightning flash rune increases your dodge by 16% for 2 sec
• Keen eye rune for the deadly reach primary which increases armor by 50% for 4 sec

The other reasons would probably be that most monks use a shield (increased armor, resistances, block% and blok amount) ant that most monks use some form of life stealing in some degree often combined with increases to att speed.

All these are some of the most common reason, there are of course others.