Originally Posted by zze86

There are a couple of reasons:

a) Without getting too technical about it, the crankarm's length has less impact on the overall system than the gearing. If you want to get geeky about it, I would suggest reading up on gain ratios first and then play with the
gear calculator via the late great Sheldon Brown's website.

b) chainrings and sprockets are cheaper and easier to change out than a crankset

c) the selection of crank arm lengths are much more limited than chainrings/sprockets although selection for single speeds is much better than multi-speeds.

Also you may want to re-read my other post. I must have edited the post while you were replying.