Track drops just look the most 'natural' on track bikes...for obvious reasons.

Road drops offer a grand variety of hand-positions, most of which are 'correct' ergonomically-speaking.

A flat bar is nice for leverage and improving an upright body position. (It also has inexplicable appeal among the white-belt-and-ironic-moustache demographic.)

For the life of me, I can't see the value of bullhorns or aerobars beyond being extremely aero in a TT.