One thing is packaging: there's a desire to have a single file to ship to users to put on their devices instead of a whole folder (and changing that at this point is probably too painful to be worth it), and then you need to address how to deal with addressing (in URLs) other things within the package.

Of course, being packaged also make it much easier to apply DRM (and, perhaps thankfully, means we don't actually need to address DRMing of HTML content directly which helps fight against it being in browsers).