Comment by worble

Possibly that for Ladybird to move forwards more quickly, it needs access to OS API's or Graphics stuff that Serenity can't yet provide? (I'm just spitballing here, I don't know much about OS development)

To take it a step further, due to the fact that Andreas is being sponsored to work on Ladybird full time by a few companies, if it's inherently also tied to the development of Serenity (since its a primary target) that might make him concerned that there's a clash of responsibilities there - he might not want any sponsors pulling out or arguing that he's spending their money on Serenity instead of Ladybird.

This is all just assumptions however, and regardless of the reasons I think he's doing the right thing.