This happens with other abilities on other jobs as well. At a new expansion, they look at what a job doesn't have that other jobs in the category do have and add it.

So some jobs get an AoE version of an ability later than another job, or a gap closer later than another job, and that's how it is.

One example was how Paladin spammed Holy Spirit like Warrior spams Fell Cleave. Then they added Confiteor to Paladin. In EW, they thought, let's give Warrior a version of Confiteor (Primal Rend).

But since they gave Paladin a Blade of Faith combo, we could dare to make the prediction that Warrior will get an Axe of Rage combo in Dawntrail, and that would not be available at the same level as Paladin's version.

SE just does this with the jobs so if they fixed Dark Missionary we could pose the question of why not balance the time of gaining similar abilities across all jobs as well.