Foundation is going through a gradual open-source rewrite by Apple and the Swift community, which is going to take a while but will eventually open up Swift more to other platforms.
IMO if Swift had had a great multiplatform toolchain/developer-experience in the ~2015-2019 timeframe then it could have established itself as the no-brainer C++ successor. But that moment has passed and they squandered the opportunity.
If nothing else, because the King says what tools are allowed.