I'm guessing because BoringTun has been in a state of "currently undergoing a restructuring" for something like 3 years by now, I'm guessing Mullvad wasn't too keen to maybe/maybe not be able to contribute, and much more prefer being in 100% control of their own implementation.

As someone who wants to see Wireguard succeed and in even wider use, this move makes sense from that perspective too. The more implementations we have available, the more we can trust that the protocol is secure and stable enough. Personally I also have about 100x more trust in Mullvad than Cloudflare both in terms of security but more importantly privacy, but that's just the cherry on top.