Because they're lower than blood and unholy. This is common knowledge. Each tree provides a buff as well, so I dont see any particular reason why Frost should remain where it is. And Blood or Unholy DPS is not overpowered, or blizzard would be taking steps to nerf them. Frost was doing fine way back when it had the ability to use 2 handed weapons really well. But it's been "nerfed" since the change to dual wield.
It's probably really easy to buff frost without doing too much of a pvp buff. Improved Icy Talons could provide more haste, Dual Wield Spec could provide more of a offhand damage increase. Death Chill could be changed to be a better offensive cooldown. I would even go as far as to change Improved Frost Presence to have you carry over your 15% attack speed haste from unholy presence instead of keeping your stamina.