The ancient elves failed the Dalish by remaining in selfish seclusion.
Why's that?
see...this touches too close to real world things for me. i can't honestly say i'd be cool with my human inquisitor...or any other inquisitor that isn't Dalish drinking from that well. Sure once they've had access to it, share the knowledge. But i can't go in that temple and deny them that right and access to it first. i only trust it in Morrigan's hands when I'm not playing as Lavellan.
And also, it doesn't matter what that history is...even negative...it is their history...they deserve to know it first and foremost. i honestly can't see why this is debatable. 
Because the only real link between the Dalish and an ancient culture, they understand about as much as other cultures in modern Thedas do, is pointy ears and that's just not enough of an argument to me for the Dalish to just have first crack and sole rights to a treasure.
Alexia also asks a good question: what about non-Dalish elves?
Cat made a valid point: This is the equivalent of finding an ancient library of pre-modern France in France and believing you have sole right to it because your French.
It's also exactly why Egyptian Islamists have no right to threaten to destroy the Pyramids of Giza or Afghan Taliban members to damage ancient Buddhist temples in Afghanistan just because they might be Egyptians or Afghani's who do the destruction. When they do that they destroy a part of the heritage of all of mankind.