I write for businesses, and I think a lot of us probably can't mainstream because either we can't handle a large load of customers suddenly pouring in (although it's probably our lifelong dream), or we don't want to take the chance in hiring more labor and hope everyone is as devoted and committed to the company's success as we are. There's probably a lot of other reasons, but those stand out the most to me. I certainly can't see business writing as becoming a "factory" type industry anytime soon, with those conveyors of word-smiths standing around waiting. :-)