One simple reason is this:
With 1... c5, you're keeping White fron establishing a pawn center with d2-d4, and the only cost is a small lag in development. White has no intention of playing an early f2-f4 as it is committal and sometime weakening.
With 1... f5, you're making a rather committal move in order to prevent White's e2-e4 when White can play c2-c4 which is almost as useful.
See the difference?
A fairly sensible set of questions. I quess there will be people who will disagree with you about your statements as to the quality of the Dutch...etc...etc....I am even sure Pfren will show to gives us his pearls of wisdom. After all he seems to have an opinion on everything...
My two cents for what it's worth is that perhaps it is more of a trend thing. Just look when Kasparov played the Scotch against karpov and the whole chess world wet themselves. Maybe no one plays the Dutch because no one of any note has chosen to champion its worth?