Most developers package all version together; plus patches. One would have to have market products for each version and users would not get free upgrades to modules.
What? Look at some market downloads from the top vendors. Often one zip, inside two or three zips for different versions and then updates and sql updates etc. No, not simple to just upgrade. They would have to have a single zip that contains only one version packaged so that the updater could use it.
It's a shame only a few developers use it.