I've been seeing this talked about a bit. To me, there is one main reason RDPS is less played than melee and it also plays into why there is a shortage of tanks and healers in group finder as well.
Melee have 12 specs to choose from spread over 9 classes. Of those, only 4 specs are on a pure DPS class (3 rogue specs plus Survival hunter). That means 75% of melee specs and ~80% of melee DPS classes can either tank, heal, or both in addition to doing DPS. If you want to melee dps and tank, you can play Druid, Death Knight, Monk, Paladin, Warrior, or Demon Hunter. If you want to do Melee DPS and heal, you can play Druid, Paladin, Shaman, or Monk.
Range have 11 specs spread over just 6 classes. Of those, 3 of the classes, making up 8 of the 11 specs, are pure DPS classes. Hunters (2 specs), Mages (3 specs), and Warlock (3 specs) can *only* DPS. This means 72% of RDPS specs and 50% of RDPS classes have absolutely no role diversity. If you want to RDPS and tank you have to play a Druid. If you want to RDPS and heal, you are a Shaman, Priest, or Druid.
The basic issue is that playing melee DPS gives you *much* more flexibility to off spec as a different role. If Blizz really wants to help address the imbalance of melee vs range and the shortage of tanks and healers when forming groups, they need to address the extreme lack of RDPS specs with tank or heal off specs. This goes beyond just adding a new class next expac that can RDPS, heal, and tank. We really need more of the current classes with RDPS specs to gain the ability to heal and/or tank.