Since BAX and MNTA do not know whether a given FoB will ultimately be approved as interchangeable, it’s much better from a business standpoint to pursue FoB’s for compounds where there won’t be a lot of other players. That way, even if the FDA deems a given product non-interchangeable, respectable sales are still possible.
For Rituxan, on the other hand, a non-interchangeable FoB in a market with five or six similar products would almost certainly be a money-losing proposition.