I've had a few informal chats with BS people and it's neither money not focus issue. While there are a lot of drawings of the locos, and they already had many drawings, each manufacturer can require drawings that match their own production capabilities. And things change: motor manufactures revise motor lines, technologies improve, decoders may need to be customized to fit new spaces, etc. Remember that these drawings are for a complicated, miniature clockwork mechanism that had to be near-perfect and easily manufactured.
Mike Conder