Actually, Dodge stated they wanted to keep the '08 Challenger SRT8 to a "limited production" of under 10,000 units. They could have made the final number 9,999 and kept their word, but for whatever reason, decided on 6,400.
Personally, I think it had more to do with the capacity of the Brampton plant over a limited amount of time - allowing for possible production problems - before coming out with the '09's.