More eyes and hands on a single open source code base is better use of resources and leads to a better output.
Historically, I've heard from various sources that Mozilla may not work at the same pace as some of the large corporations do in upstreaming patches and releases to the code base. This has (according to what I have heard, which may simply rest on rumor/speculation/heresey, mind you) kept a lot of large entities from getting on board.
I've also heard that in building Quantum this is less of an issue, but their code base is a mess, and chromium's is incredibly well documented and clean by comparison.
Anecdotally, I would be inclined to agree with my second statement up front, after just browsing the two, but please compare for yourself
https://developer.microsoft.com/en-us/windows/pwa
I also am willing to surmise that they are going to put in alot of work re-working all their APIs that are exposed as Edge based interfaces into whatever engine they and up with here (namely chromium), so I don't think it would have been beyond them to put in the work to do something like they did with node and chakra core:
https://github.com/nodejs/node-chakracore
(which, btw, for now isn't dead: https://github.com/Microsoft/ChakraCore/issues/5865 likely for the reasoning I mentioned above)
I just don't buy it. I don't think easy electron compatibility is the main drive here.
I don't really get why people judge source code by politics rather than "stealing" (forking) from the best.
Google is the main power behind all Chromium development and there have already been plenty of examples of the devs going "our way or the highway" with noone forking the browser engine. Upcoming is the new change to block adblockers from working properly in chrome. I see nobody willing to fork chromium over that.
You'd think a business would do whatever maximized value and ignore past negative interactions, but businesses are made of people.