I am confused why the answer isn't B. It seems to be parallel with traditional retailers as well as D.

This one is not an issue of parallelism. It is an issue of comparing like things. We have to make sure that we are comparing traditional retailers to fast fashion retailers. Choice D makes that clear by using the commas to indicate that " by delivering ... " is modifying fast fashion retailers.

B makes it seem as if we are comparing the "delivery of new looks" to traditional retailers.

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