The changes required to adapt to these stores ... are minimalThe changes from what, though? I've just looked through both the Chrome Store and Firefox Marketplace sites, and it's still not clear to me what they actually offer to users, or what their business model is, or in Chrome Store's case how developers even get involved (assuming they can figure out what they are getting involved in or why they care).
Both sites seem to be unclear about almost everything, including what they do or why I should care as either a user or a developer. So it's not really surprising that most web developers here aren't targeting them.