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 [/url]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.