RDP is aimed at a different use case than VNC. Proxmox and other virtualization managers (e.g. VMWare, Nutanix) use VNC because you get a stream directly from the hypervisor (e.g. KVM, ESX) which is very useful for low-level debugging. The VNC protocol also has very low overhead (you don't really want h264 encoding CPU overhead on your VM host). VNC is not really intended for remote desktop use cases, which require higher fidelity/frame rate, etc.
So -
* VNC: Low overhead / Low fidelity
* RDP (and other remote desktop protocols, e.g. Frame Remoting Protocol, Horizon Blast, Citrix ICA/HDX): Higher overhead / High fidelity
So -
* VNC: Low overhead / Low fidelity
* RDP (and other remote desktop protocols, e.g. Frame Remoting Protocol, Horizon Blast, Citrix ICA/HDX): Higher overhead / High fidelity