KaOS is getting ready to move to Wayland as the default session, current ISO in testing has made that switch.
That also means all default applications used on an ISO (thus part of the default install), need to be ready for Wayland. All current default applications are, with one exception, SMplayer. From the many bug reports for Wayland support & SMplayer (dating back to 2017), it seems SMplayer is unable or unwilling to move to Wayland, thus can no longer be default (example of a bug report & developer response: https://github.com/smplayer-dev/smplayer/issues/271).
As a replacement, Haruna has been chosen. It is developed for Plasma, is QML based, has yt-dlp support, subtitle feature and a rich set of configuration options.