At the launch of 2.0, Bard did more damage than melee DD's while having buffs and freedom to do mechanics with minimal uptime loss. As a result, many groups chose to bring a 2nd Bard and only took the minimum amount of melee DD (1) if they even took one.

Many of FFXIV development team's balance decisions have drastically favored melee DD buffs so that 2 melee or 3 melee DD compositions were desired. Melee DD's are pretty much worthless if they did not do more damage as they take a lot more time to optimize and create strategies to accommodate melee DD's. Melee DD's are generally terrible to do mechanics with and lose a lot of damage if they were not catered to.

Traditionally in MMOs, ranged DD's are just better in every way.