Without proportionality of the wheel size, proportionality of the crank length results in non-workable gear ratios on either rider-size extreme that render true crank proportionality on road bikes impractical for pro racers. Tall sprinters cannot get a high enough gear to work with a long crank, as 80kph on a 53 X 11 gear is 135RPM, a cadence that is unrealistic when the crank circle gets much bigger than 180mm in radius. And short riders spinning a crank a lot shorter than 165mm will be held back by gears that are not low enough to offer the high cadences called for by short cranks on steep climbs. And then there’s the problem that pro riders no longer ride custom frames; they ride cookie-cutter carbon frames coming out of molds, so their bottom bracket heights and seat angles cannot be adjusted for extremely short or extremely long cranks.
Read more at Technical FAQ with Lennard Zinn: When it come to crankarm length - no easy answers - VeloNews.com