Easier for the manufacturer to make their FDs work on 28.6, 31.8 or any other diameter tubing, I'm thinking. With the various sizes of steel, OS steel, Aluminum, carbon composite and what have you, it makes sense to make one model of FD and a bunch of different bands to fit it on multiple different seat tubes. I'm guessing Shimano was mostly prompted by the transition from MTBs going from the standard steel tubing sizes the the new, fatter aluminum sizes.
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