I don't have any more insight than any other commenter, but in my estimation a major factor is how practical the browser is to fork. By the time Microsoft switched to a Chromium base for edge, creating and maintaining a Chromium fork with meaningfully different UI was fairly well-trodden ground because it had been done several times already, whereas almost nobody had forked Firefox (except for toggle some flags or keep the UI frozen in time). The one countervailing example, Brave, also switched to Chromium for similar reasons.
Additionally, this was the beginning of the arc of working overtime to court web developers that it's still in the midst of. By shuttering Chakra (the old Edge rendering engine) and switching to Blink, Microsoft improved its reputation with web devs.
My 70 year old mother doesn't want the faff of installing Firefox so Edge fits the bill. It provides for her, her needs. I've installed Firefox and it sits untouched.
Microsoft doesn't care if people use it or not. It's easier and cheaper for them to integrate as Chromium does than it is to upkeep Trident. It's not their business too.
My take to why they chose Chromium is that Firefox (Netscape) has always been seen as an independent rebel.
Microsoft is corporate as is Google. I'm sure some backhand deals too.