In the Ubuntu 17.10 cycle we tried wayland as the default session but we didn’t feel confident at the time it was ready yet for a LTS. Things improved since, some of the blockers we found back then got resolved (desktop sharing), and that’s where the upstream focus is going. We believe now is the right time to try again, it should give us enough time before the next LTS to get proper feedback and sort out issues.
Since we decided to not upgrade our GNOME version this cycle it should also make things a bit easier. Note that nvidia users are still going to default to xorg for now but hopefully that situation will be resolved before the LTS.
We will keep this topic updated as needed, as usual community feedback is welcome and bugs are best reported on launchpad.