Melee as a whole is weak right now, but most of the other melee have secondary roles. Warriors, paladins, druids, monks and DKs can tank. Shamans, paladins, monks and druids can heal. Shamans and druids can caster DPS. We have only one role, and that's melee DPS, and with melee being weak overall compared to range, it's not surprising that we're on the lower end of the played spectrum. Also, the class hasn't changed much since BC, it has a stale rotation and nothing particularly interesting between our specs (mages and warlocks change a lot by spec). The best class to compare us to is probably hunters as we're both stale, pure DPS roles. Only reason hunters aren't lower on the played charts is because hunters are more viable overall (stronger in PvP, easier in PvE because ranged>melee).
Blizzard's admitted that the melee niche of being able to damage on the move is gone with all the on-the-move casting that ranged has gotten, so hopefully they'll either stop making fights so punishing for melee or they'll give us another niche. Would also be nice if they FINALLY addressed issues that Rogues have been complaining about forever.