S9 will be in the market for a long time. If you are Bitmain, why would you want to release any new product when existing product still hit a 3 month ROI...
One reason may be to stay ahead, you don't want to find yourself 6 months from now in a position when a competitor is selling equipment you can't compete with. Also, there are players like Bitfury that may be eating part of Bitmains lunch in the industrial space.