Right now, there are two worlds of fashion in SL.
In one, vast numbers of people slave over textures and prims to reproduce the subtle details of the way light plays on the textures, folds, and wrinkles in different kinds of cloth to create amazing fashions. (OK... to the extent that's possible in a world where skirts are made up of a bunch of rectangular strips tied together at the top...)
Even the high end products are not all that expensive, because the makers can take full advantage of economy of scale and a huge market for their products, which all are made for the standard issue avatar form. Anyone using those avatars can wear any of that clothing... and there's so much of it! The list of SL fashion blogs, both those from designers announcing their own products and from reviewers who cover that portion of the cornucopia they can manage to keep track of and find worthy, is endless.
In the other, individuals piece together prims to cover avatars or body parts themselves built from prims. The need to do that makes it very difficult to do anything other than solid colors; very few patterns or textures here. No economies of scale here, either; these people are making clothing to cover the avatars or body parts they themselves have made, thus splitting the time and resources they have to devote to each, and they will at most sell as many copies of the clothes as they sell of the corresponding avatars or body parts they are designed to fit.... and the market for those is diminished by the nagging thought at the back of the shoppers' heads: Look at all I'd be giving up! All the dresses, uniforms, swimsuits...
Do I have a solution? Not really... but the makers of specialized avatars and body parts not supported by the stock avatar model really should try to come up with some way around it. Coming together as a group to work on the problem sounds like a good first step to me.