Because there should be.
It's crazy that everything still wants to put stuff on C:\ ... I'm looking at you Visual Studio and Win SDK.
Plus Users\Public shared goodies. And the %userprofile%\AppData. I get that every application thinks it needs to put its cache/files there, but then why can't folks relocate users' homes? :/
I'd love if that team would also take a look at Android, where you simply can't not install on the "system drive" aka. main phone flash.
Doesn't hurt that it helps speed up the upgrade cycle since users run out of on-board space sooner.
In how many places does Windows concentrate the settings for everything?
Makes sense to get rid of one of those, especially as restoring a registry backup can lead to unintended issues if you've installed something since, whereas a system restore point takes care of all of that for you.
50MB here, 100MB there ... pretty soon you're talking real storage usage.