One of the major advantages of developing for the Web is the platform neutrality. You can rise above OS wars and browser wars, and provide something -- one thing -- that almost everyone can use whatever other choices they make.
Given the above, why is there any advantage for a developer in having a platform-specific store for web apps, and why would any user want to access web-based software via a store instead of just going to the site? What is the value proposition here?
The only thing I can think of is marketing, but I'm not convinced they offer much benefit there. Despite being a regular user of both Chrome and Firefox since they were new, not to mention a professional web developer, I don't remember ever hearing of Firefox Marketplace before this thread and only just read up on what it is. Although I'd seen references to Chrome Store when starting Chrome, I had immediately ruled it out as clutter that didn't belong in a browser and hid as much of it as possible so I could go back to visiting web sites. Certainly no client has ever asked me about using either of these channels for a real project.
Of course this is all just personal anecdotes, so it's possible that I'm an outlier. However, it seems like these channels aren't exactly Apple's App Store where most people get all the software they run on a given device. So I come back to asking why any web developer would develop for Chrome Store or Firefox Marketplace. What's in it for them?
The 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.