It only explains what is different between sudo and runas. It doesn't really explain why they couldn't just improve runas and add those missing features instead of creating a new command. As it stands now you seem to need both sudo and runas to cover all reasonable use cases.
Backwards compatibility and changing the security dynamic of existing scripts. I'm sure it is probably possible to achieve using runas, but my guess is that it would greatly delay adding this functionality, as they would have to cover all concerns for which runas is used.
