its more due to what the content is, since most use it for levelling from 50 to 60 tanks and healers have a easy time with the 3 lvl 50 raids, LotA and CT are very easy heal wise and all 3 are very easy tank wise, so with that the DPS in need comes up more its the same with main story roulette, since people don't want to wait over an hour to do one dungeon that one also tends to get a lot of DPS in need at times.

change the roulette to have the lvl 70 ones included and you'll probably never see the DPS in need for them for a while.