It might be sort a kind of a contract thing.
When they started out, both Inoue and Miura didn't really have the absolute working freedom they have now, and were compelled to release chapters on schedule.
But as the work proved to be lucrative, they paradoxically were allowed to slow down their output and were given more freedom by the editors and publishers. Which, for perfectionists like them, meant drawing and redrawing scenes ad infinitum.
Also Inoue, mangaka of Vagabond, has a second work he's simultaneously doing called REAL, which is probably eating up some of his time.
I know pic related isn't /co/ but the work should be read by everyone.