I don't think there's a reason for it other than it is just the way it is.
Some games you fly by pushing up in mid-air. Others you use 214K. In this one, it is D.
I haven't found it to be all that unintuitive (just push the buttom). I do occasionally still get a dive kick or something when I'm holding a direction, but that's just mechanical control, and it is almost moot at this point.