Well,
If I had to give an answer I would say, since main racebooks are still coming out, not to mention storyline books with rule additions, etc it is hard to publish a game screen that would include everything that needs to be on one. Not to mention if they had created one after the main book it would be WAY out of date.
For that reason, I personally would NOT like to buy a new gm screen every month, give mongoose time, let them do it right.
PsyJack