The Ubuntu Developers made the decision to migrate to Ubuntu 20.04 using the GNOME user environment which by default runs on the basis of the Wayland protocol. Using proprietary NVIDIA drivers will continue to offer an X server based session by default. For the rest of the configurations, the X-based session will be mapped to options.
It is noted that many Wayland-based GNOME session limitations have recently been addressed, which have been identified as issues blocking the transition to Wayland. For example, it is possible to share the desktop, using Pipewire Media Server.
With Ubuntu 20.04 abandoning the move to GNOME 40, there is a good reason to focus the new release on Wayland integration. Either way, there is still plenty of time for the next LTS release, scheduled for April 2022, to resolve issues or roll back to the default X server session.
As a reminder, Fedora has been using the Wayland-based GNOME environment by default since release 25, published in 2016. Wayland is also enabled by default in Red Hat Enterprise Linux 8. In Ubuntu, an alternate Wayland-based session has been offered for testing since the 15.04 release (April 2015). In 2017, the first attempt was made to migrate the default Ubuntu 17.10 release to Wayland, but Ubuntu 18.10 brought back the traditional X.Org Server-based graphics stack.