Warlock is relatively hard to play and is somewhat less rewarding, especially in PvP: low mobility, mediocre survival, easily counterable CC. Also, in PvE it can fit only a DPS role, which makes the class pretty straightforward and less versatile than, say, Shadow Priest, a hybrid spec being able to offheal, especially in low and mid-level dungeons.
All of the above made Warlocks one of the least popular classes - not only here, but on retail servers as well.
Rogues, on the other hand (despite being a solely DPS class as well), are quite decent both in PvP and PvE, but it's the hardest class to level in a no RDF, no loom condition. Rogues can't heal at ALL in Wrath (except bandages), so they have hard times to kill elites, deal with packs of mobs, surviving at low and mid levels overall. In PvP rogues got hard times vs. plate classes, and since plate classes are the strongest in Wrath PvP, it also has its influence on Rogues' population.