So, what's with the lack of Machinist in DF?

[Docent]

I started leveling my own Machinist. The Dps I normal see are in this order:

Well, the game actually matches ranged DPS with melee DPS, so you'll see Dragoons and Ninjas the most when on your machinist. Monks aren't as popular by a big margin.
So next, you're going to see Bards, Black Mages and Summoners, because those classes are very popular, despite being the same "type" (ranged DPS -- For normal Duty Finder, the game doesn't distinguish Caster and Physical ranged DPS) as you.
Finally, since the game rarely pairs two people of the same class, you'll see Machinist the least.