OpenSSH is an implementation, SSH is a technology and/or protocol, it doesn't make a lot of sense to compare them that way. Although from later comments RDP seems to have a history in Citrix prior to it's life as RDP, so there are likely clear reasons why it didn't use the SSH protocol (namely that the underlying technology RDP was built on does predate the SSH protocol).
The Citrix/Microsoft relationship is a bonkers story. The world didn't yet know that licensing your tech to Microsoft meant you were going to grab your ankles.
Smart cards is just certificate-based authentication really.
Microsoft did indeed fail hard - if they just made certificate authentication easy to configure on stand-alone machines like SSH key authentication is (and allow to keep the key as a file if desired, instead of in a smart card), a lot of pwnage would've been avoided.
You mean seamless integration with AD and supports smart cards?
> makes me wish they just went with SSH as transport protocol.
RDP predates OpenSSH by a year.