I think it is the publisher. No offence to FFG but regular bookshops like Borders or Waterstones already had books from Black Library so it was no real problem for them to add these items to their stock and bookshelves next to the WH40K novels.
FFG on the other hand doesn't appear to have the same relationship with the book sellers. They do the FFG stuff as a special order.
So I would say the hassle of getting the DotDG was just a slight downside of the move from BL to FFG.