Windows

There's an overabundance of competent-ish frontend developers. You most likely need to pay the devs less, compared to someone writing it with e.g. C++, and finding people with relevant experience takes less time. You also get things like a ready-made sandbox and the ability to re-use UI components from other web services, which simplifies application development. So my guess is that this is done to save money.

Also, the more things are running in an embedded browser the more reasons M$ has to bake Edge into the OS, without raising eyebrows as to why they're providing it as a default (look it's a system tool as well, not just a browser).