Microsoft would have to consider the long term: What will Chromium and Gecko/Firefox look like in 5 and 10 years? Which is more likely to provide the technology that Microsoft expects to need? Microsoft can't just switch engines every couple of years. Some back-of-the-napkin analysis:

* Vendor stability is one issue: While I doubt Mozilla will go out of business, Google clearly is a very safe bet.

* Which vendor will invest more in keeping the technology up to date? Google, due to its vast resources, seems like a safe bet to invest more money and engineer hours. Mozilla, however, will certainly make the web browser the center of its attention and top priority; Google has a lot on its plate (and Microsoft knows how browser tech can drift for years without top management attention).

* Which browser is most likely to maintain compatibility with websites and web apps, including corporate ones? Google is the obvious choice.

* Which vendor's engineering priorities most align with Microsoft's? I'm not sure. Google has their own priorities - a front end for Google's services, which compete directly with many of Microsoft's - but Google seems more focused on the corporate market. On the other hand, Mozilla's focus on privacy and end user control could easily conflict with Microsoft's priorities.

* And of course, which browser fits best in Microsoft's immediate browser plans and has the cleanest upgrade path from Edge/IE? I have no idea.

I'm a dedicated user and supporter of Firefox and Mozilla, but I can see reasons Microsoft might choose Chromium.