Dragoons have been a historical job in the Final Fantasy games, add a dragon pet and cool armor to that and I can see why a lot of players want to be one.

I'm sure Dark Knight attracts some of the darker brooding personalities out there, and with teenagers as your main demographic there are a lot of them.

Plus they do a lot of damage and I can see how more people would want to kill kill kill with their weapons than take damage/heal/support/nuke.