Denim jeans were originally worn not so much as a fashion statement as

We have two choices of structure here: "not so much X as Y" or "not X but Y."

B-D mix these two structures together: "not so much X but Y" or "not X as Y."

A and E are both miserable and awkward, and as a writer, I wouldn't want to sign my name to either of them. BUT . . . A has two advantages over E:

First, the parallelism makes more sense. A compares "as a fashion statement" with "for their being practical work clothes." These are both modifiers, and they can be parallel, even if the second one is terrible. E compares "as a fashion statement" with "as for them being practical." This doesn't make sense: they're worn as for their being practical? (Note that this is different from A, in which the "as" is part of the comparison structure: "not so much X as Y."

Second, there's a difference between the use of "their" and "them." In A, the possessive pronoun "their" modifies "being practical." In E, we have the regular pronoun "them." This isn't a modifier, so it doesn't work to convey the desired meaning. This is a common grammar trick that contrasts with how we often speak in real life. You might say "Do you object to me leaving early?" but we should really say "Do you object to my leaving early?" It's the leaving early, not you, that they should be objecting to.