Microsoft must have realised that picking Chromium would dramatically increase the odds of a Chromium-only web? On the surface I'd have thought it would make more sense for Microsoft to support the Mozilla Quantum project. Perhaps they just wanted to take the easy route though. Chromium is certainly more popular and has Google's backing. I understand why so many projects use it.
Mozilla killing off support for embedding Gecko in other UI toolkits was a massive footgun, even if they didn’t realize it at the time.