Doesn't it seem odd that cranks typically range from 170 to 175 mm on most bikes. A mere 3% variance, yet rider height and femur length vary by 30%.

21.6 percent of inseam for a tall rider equates to a 200-205mm crank whereas a short or short femur rider might need 145mm crank using the formula in the following websites. Only a few mfgs make even 180mm cranks and none of the big names make cranks shorter than 165. I am experimenting outside these ranges and it ain't cheap to do so. I think unavailability (long and short cranks) and lack of good data supporting proper crank length is why everyone just rides 170, 172.5 or 175 for the most part.

Bicycle Crank Length
Technical Q&A with Lennard Zinn: A question of crank length - VeloNews.com