The rumour i heard is they were making their own custom thing.

There was some rumours that they were unhappy about mediawiki's response to patches they submitted (they made a bunch around accessibility). However i looked through their patches at one point when this rumour started flying around and it looked like most were merged. Those that weren't generally had code review comments with questions or pointing out mistakes which were never replied to. I sort of suspect the patch thing was some sort of internal excuse because the team involved wanted to make their own thing.

Regardless, im really happy they decided to open source their extensions and it was nice to see that they put in effort to upstream core patches.