I'm still sad they didn't partner with Firefox instead. It would suit the strategy of The New Microsoft very well to ensure competition in the browser market, and making an Firefox-based Edge plus a Firefox-based Electron clone would've been an amazing way to do that. They've shown competence in embracing, extending, and contributing back to OSS (eg they were instrumental in Nodejs getting to its amazing cross platfo…
Firstly, I'm paraphrasing here, but FF isn't as much a business as it is a religion (re: privacy). You can read a lot in to that, but I understood the gist of what they were getting at.
Secondly, somewhat relatedly, but consider FF's primary funding/revenue: it's Google. MS building on FF could mean Google would reduce their search deal, meaning MS might end up being forced in to increasing their funding for FF to help keep it an entity capable of operating at its current size. Or, FF simply becomes smaller, and MS has tied themselves with a shrinking partner.
However you look at it, MS has not had a good run in promoting their own browser tech against competitors, and ends up losing share, so why fight that?