This might be known already, but I bet that Microsoft decided to switch Edge to Chromium instead of forking Gecko/Firefox because Google either bribed them or threatened to lower Microsoft sites' ranks in search results.

Otherwise why would MS use a web browser controlled by one of their very few competitors?

Edit: maybe they were enthralled by the promise of using Proton/Chromium based "desktop applications" (which just contain an entire Chromium browser in their install directory) to cheaply create apps that people are forced to use in their jobs, like Teams. Which is still awful even after they made it a full UWP desktop app. Like Skype already was.