As mentioned elsewhere, the use of significantly longer cranksets needs to be accompanied by framesets with a smaller drop -- raising the BB higher off the ground to maintain cornering clearance. This may not sound like a big deal, but it is. Framesets and cranksets are typically made by different companies. Therefore, a crankset maker cannot sell a long crank because there's no frames designed to use it, and a frame maker cannot sell a frame with a high BB because there's no cranks available to use with it.
If consumer demand warranted it, these problems would be fairly easy to overcome. However, to quote Bob Morris: "There cannot be a demand until there is a supply." If the consumers are unaware of the advantages of properly-sized cranksets, it's unlikely the manufacturers are gonna hear any demand to change their ways.
In the past, the problems were even worse. Up until a couple of decades back, all high-quality bicycles were made with butted tubing brazed into cast or forged lugs. Often, the lugs and tubes were made by different companies and assembled into a frame by a third company. When the BB height is to be changed, the angles at which the seat tube, down tube, and chainstays attach to the BB must change. This requires a different lug forging for each BB height. Since the use of crank lengths according to this formula would require significantly different BB heights for each frame size, different forgings for the BB shell will be required for every frame size. While this might be reasonable today, in decades past it would have been considered an outrageous expense -- and clearly unwarranted if buyers were not demanding it.
The same problems existed in crankset manufacturing. In this day of computer-controlled machining and advanced aluminum forging processes, the production of different length cranks would not really be that big a deal. But in the past, making a high-quality forged aluminum crankset with integral spider in several significantly different lengths would have been considered prohibitively expensive.
Aside from the manufacturing and market issues, the various cycling team coaches seem to have done their part to ruin the reputation of differing crank lengths. Coaches seem to regard variations in crank length as a "speed secret" rather than good bicycle fit. There have been a few scare stories of coaches who have forced their riders to use cranksets that were too long for them, resulting in damaged knees, shortened careers, etc.
For the most part, coaches don't appear to have put any more thought into proper crank sizing than anyone else. If a rider from some other team did well with a long crank, they'll try it with their riders -- regardless of size. And if someone blames knee problems or poor performance on the use of a long crank, they will never again consider long cranks -- even for their tall riders. Such is the way tradition is developed; it has its place, but it needs to be tempered with some sound engineering practice as well. Over the past few years, some of the bigger racing teams have developed a better attitude toward trying new ideas, so perhaps rational crankset sizing will eventually come into common use.
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