Same reasons that IE versions only support certain versions of Windows:
- they want to be able to switch to newer APIs when the underlying OS adds them (though in Edge's case it's more likely it was written from the ground up using newer APIs)
- they quite possibly want to use "you can get the new browser only if you upgrade" as a carrot for OS upgrades (they explicitly did with IE, I haven't seen anything explicit for Edge but it wouldn't surprise me if they're still taking that approach)
- they want to be able to switch to newer APIs when the underlying OS adds them (though in Edge's case it's more likely it was written from the ground up using newer APIs)
- they quite possibly want to use "you can get the new browser only if you upgrade" as a carrot for OS upgrades (they explicitly did with IE, I haven't seen anything explicit for Edge but it wouldn't surprise me if they're still taking that approach)