Few factors..

Just the way tanking is now its about kite, snap aggro, quick in and out to get aggro, and more kiting. DH is just designed to do that more or less. Other tanks can do it it but..

Then you get to the community level thinking. MDI was like only DH so every loser is like.. DH only.. and then once you get super meta (like +20 certainly is) it almost becomes necessary. Again, sure.. other tanks can do it but when you are super meta.. you dont say.. well.. its only 4% less efficient. No, you say fuck that 4% and take it. So it makes sense from the meta perspective and is retarded in say a 7 when its like DH or bust.

Does it mean the current way the game is balanced is bad? Well.. kind of.. but at the really high end it will ALWAYS be this way. You will always have a best and at a certain point meta will dominate because that 4% (or whatever it is) matters. Really it is only screwed up if it has to be that way at around the 10-15 range because that is where you shouldn't have to be super meta, just good enough to get it done (not to say meta doesn't still help). So when you say 20+ keys are dominated by a certain class.. well.. that is just how it is going to be. When you start to see it in 10-15 then I would start to worry. Now they will be higher because copy pasta folk will gravitate to it but if it ever becomes like 80-90% then they forsure got to look at it.