Regardless of class, there are basically two types of D3 character builds:
1. Very low DPS, "tank" builds used by undergeared characters so they can actually clear an Act in Inferno without bind zerging the elites. They stack as much survivability as possible and bore the mobs to death.
2. Very high DPS, "glass cannons" builds used by geared characters to maximize farming efficiency. They ignore survivability stats and abilities, and stack as much DPS as possible in order to kill !@#$ fastest to farm the most items. These builds survive via lifesteal/LoH and using their massive DPS to outheal the incoming damage via lifesteal.
Monks are bad at the first type of build because their heals don't scale. They get whittled down to nothing if they have no DPS.
Monks are also bad at the second type of build simply because Monks have the lowest DPS of all classes. This is mainly because crit/crit damage is on the same items that can roll high DEX, which makes it hard to get both tons of DEX and crit/crit damage.
Monks have a ton of other abilities to make up for their low DPS, but the main issue is that, regardless of what class you are, the only stats that matter for endgame Diablo 3 are DPS and movement speed. That's it. Nothing else matters. As I mentioned, endgame builds tend to be extreme glass cannon builds that stack nothing but pure DPS stats and life steal. They usually have something ridiculous like 20-30k HP and maybe 200 allres. If they stopped attacking, they would die in two seconds flat. However, it doesn't matter as the monsters can't outdamage the life steal that 500k DPS characters put out.
However, even having lots of LoH and LS is not important because a deficit in these stats can be made up for by simply having more DPS.
In addition, the glass cannon nature of endgame builds also destroys the viability of abilities that would otherwise be very powerful. For example, with 40% crit, two attacks per second, and 200 life per spirit spent, the Quickening rune alone is equivalent to 2400 LoH. 200 LSS is NOT hard to get right now because people don't respect the stat at all and price it as near-worthless. If you count the base spirit regen from the actual attack you're probably looking at 3600+ LoH equivalent, without having to buy a single piece of expensive LoH gear. So why isn't it used? Because the randomness kills you when you only have 20k HP, no shield, and no resistances. Miss a few crits and you're dead.
Monks have the lowest DPS, so they are the worst endgame class. It's really that simple. Their other advantages are irrelevant, because DPS is the only thing that matters. The lower DPS also means they need more LS/LoH to survive, which further decreases DPS as now money/stat budget must be spend on those stats that isn't being spent on DPS.
Simply buffing Monks doesn't fix the problem. Someone has to be last on DPS. That class will then be terrible.
This is also why Monks are an excellent Hardcore class. When you can't just mindlessly stack DPS regardless of what class you are, and have the EHP to ride out random fluctuations, Monks become extremely strong.