HTTPS would be overkill. TLS would require certificates and nobody's going to put up with that for simple network transparency. It already works over SSH and that's pretty much optimal as far as most of us are concerned...

Some form of encrypted channel, already deployed everywhere and well-understood? Like SSH?

You don't need encryption for a local protocol, and wayland does not support remote desktops (yet?).

And there is not reason to use HTTPS for Wayland instead of a lower level protocol.

SSH is mentioned numerous times as a transport protocol. SSH, in case you were unaware, is implemented using TLS.