Because nouveau may conflict with Nvidia driver and load instead. You can try first switching to Nvidia only (from hybrid to discrete) in bios. Try rebooting and see if it works.
If not, here how to do it in Grub.
If you use systemd boot (default on recent eos Cassini iso), I don’t know how to proceed.
Backup your stuff first. If you boot to black screen, no panic, you can undo this and/or always switch to TTY and troubleshoot things. Additional infos…
Remember, Nvidia and Wayland do not work well together if at all.Thats why on Pop OS for example you are automatically brought into Xorg session at login with Nvidia graphics.