I can imagine devs for free launchers don't have 2k sitting around to buy the latest and greatest phone each year to update their launchers for compatibility with the latest Android APIs etc.
The standard Android development tooling includes extensive virtualization technology. So, on your dev PC, you can launch a program, which pretends to be Android hardware, then an Android OS is installed into there and then your app.
In that way, you can easily test with various Android OS versions, without buying tons of hardware.
Having said that, if the devs don't earn money, it may of course still be the case that they're not motivated to fix issues for an Android version they don't have.