The reason is that well... they don't do enough damage fast enough even in the greatest PVP gear. Very limited AOE attacks, very limited cooldowns. In cata they will improve BUT the thing about them is that they will require lots of micro management especially with all the procs you will need to deal with.

Procs to deal with
Obliterate to trigger free howling blast (45%)
Runic power from auto-attack (45%)
Killing Machine (frost strike etc insta crit)
Frost strike replenishing one random rune (45%)
Death coil replenishing one random rune (45%)
Free death coil (15% from auto attacks)
Pet energy, amount of death coils used for pet infusion.
Keeping track of death runes (as they can even replen with the 45%)

IMO its a nightmare to deal with any TWO of those combined... let alone playing 4 frost DK's. As both frost and unholy there is NEVER a time you aren't pressing a button it seems. DK is going to require a lot of skill to play well IMO. They do a shitload of damage (in mediocre gear my frost strike crits for 12k, same with howling blast and oblit) So in a few globals you can obliterate someone very easily as a DK (pun intended) but to use them effectively it will require a lot of skill. Same as paladins.