Almost nobody can efficiently DPS during Massacre, including melee, and especially not on heroic because Massacre pretty much one-shots without defensives. On top of that phase 3 has absurd movement for melee and requires the entire raid to soak outside of melee range, resulting in extremely bad uptime while ranged can just DPS during most soaks.
The top specs for heroic Denathrius are still boomies, affliction locks and fire mages (like on pretty much every fight in CN), followed by melee specs that either have heavy DoTs, ranged attacks, high mobility or all of the above - assassination rogues and unholy DKs, with fury and ferals trailing behind them.
Youâre either playing a sub-optimal spec for the fight or just playing sub-optimally in general. Whichever it is, the cause of the problem is you and not ranged being hard to play in CN.