Why this hard switch from Less to Sass?

I’m wondering why in v9 there is a hard switch from Less to Sass: Old themes are (still) forced to use Less, new themes are forced to use Sass.

Although there is in theory an internal abstraction of precompilers in the core code (as far as I can see), this is not used.

Neither is it possible to include a Sass file with the old

$html->css($view->getStylesheet('main.scss'))->getAssetURL()

nor is it possible to create new skins with less.

It seems that there are now just two complete seperate implementations, and I’m wondering why developers are forced to one CSS precompiler depending on the version of the system?

Am I missing something?