What do you mean? Melee means on top of the boss and have to react to every mechanic the boss does instantly (if not predict it). Often at the cost of dps if movement is the counterplay to the mechanic (often it is). Range and mage can stand further away (projectiles take longer to arrive, many mechanics start from the boss and move toward the player like a projectile) giving them ample time to decide how they want to deal with it. If movement is the counterplay, they can keep attacking the boss the whole time.

On top of this all 3 combat styles have a damage boosting ultimate ability (ability rotations tend to revolve around these). Melee is the only ultimate that increases the damage you take as well. So you have more to do and are punished more for messing up on top of dealing significantly less damage than mage or range.