The same reason slow weapons are preferred for any other instant strike ability -- they hit harder.
If you have 2 weapons with 200 DPS each and one is 2.6 speed its average damage will be 520. If the other is 1.6 speed, it's average damage will be 320. So even though the attack power is normalized to a 2.4 coefficient for both weapons, you'll still be getting an extra 200 damage per obliterate from the fast weapon. The same effect applies to your offhand damage as well, although you don't get quite as much from it because it only does 57.5% (with talents) normal damage.
Actually this affects almost every attack you use as a frost DK although it affects obliterate the most because obliterate has the highest weapon damage coefficient. Blood strike, frost strike, and plague strike are all affected by weapon damage as well and you'll be losing quite a bit of damage throughout your entire rotation if you use fast weapons.