The only reason MS produce a free web browser is to direct users to ads and services they control to try and monetise the end user. To spend money on developing your own rendering engine just to do this does seem like a waste from a commercial perspective. So if you can just use a already existing open source engine with your own skin on top you can achieve the same result for less costs.
Most users of Edge probably wouldn't realise that the underlying rendering engine had even changed. They just use it because it has the 'e' logo which is what they have always used to connect to the internet since the days of IE.
Biting the hand that feeds IT
