For me, it’s Shared GPU memory.
I am unsure if the specs bear this out, but my personal experience has been that RDP’s compression and encoding leads to much smoother interactions with the remote machine, especially when there are a lot of windows or visuals on screen. My bandwidth utilization has been lower on VNC.
Using RDP I also meet CMMC guidelines, which is probably doable with VNC, but not as easily or without some additional work on my end to prove compliance. It’s also easier to convince my clients to allow me to work off-site using RDP as a trusted secure protocol. Less headache.