My guess is that without the mid-rib the bay doors would have to be locked on to each other. It is difficult to achieve positive locking between 2 movable surfaces especially with drag on the bay doors. It takes a longer & it would also require very complicated locking mechanisms.Does not having a mid-rib bang in the centre of an already cramped IWB limit your choices of weapons? Since it essentially partitions the IWB into two halves. Wouldn't it have been better to mount the hydraulic piping on the IWB roof scattered around the weapons mounts (& do away with the mid rib) to be able to fully utilize the space on offer? Surprisingly, even F-22 & J-20 suffer from the same "compromise":
The opening-closing-positive locking cycle for the AMCA's IWB is supposed to be <2 seconds. That would be very hard to achieve without the mid-rib. Of course opening the bay of a stealth fighter increases the RCS, so you would want the cycle time to be as low as possible.
Just a guess.