It would appear that Unity is causing all kinds of problems for would-be Nintendo Switch developers. The game engine Unity did support the previous console, the Wii U, but is having some issues when it comes to the Switch. Which means Yooka-Laylee has become a victim.
The developers of Yooka-Laylee, Playtonic, were waiting for a new version of Unity in the hopes that would solve Switch problems they were having. Version 5.6 of Unity is out now, but it's causing all sorts of other problems with Yooka-Laylee's Switch development. These issues between have to be resolved before Playtonic can submit it to Nintendo and announce a release date for Yooka-Laylee on the console.
Playtonic released this news in the hopes that it would help fans understand why Yooka-Laylee does not have a Switch release date yet. They are working as hard as they can to solve all the issues that the game engine has as quickly as possible.
With this new version of Unity released, hopefully Playtonic will have a release date for the Switch version of Yooka-Laylee soon.