The main advantage is that we can ship mintty bundled with Julia (adding only a few percent to the download size), so that we can use it by default on all supported Windows machines. It will be some time before we can rely on the new Windows Terminal being installed by default everywhere (at which point we can switch over). The goal is to have a good user experience out-of-the box on Windows.
The most persistent complaint that a new terminal addresses is better Unicode support, because the Windows console does not support fallback fonts.
(You can still use Julia with Windows Terminal if you have it installed.)