Because you need room for the wrench on a single-bolt seatpost. Not about fit at all... I mean, if it were, wouldn't bikes just come with slightly slacker seattubes? It was either a setback or a cutout, and cutouts are harder to manufacture. Or you can do the whole two-bolt thing like Thomson and Campy, but these are more expensive solutions as well.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter