If you've extended the flaps due to a stall warning then the instinct will be to raise them once out of that condition unless you are following a procedure which specifically tells you to leave them extended. Had the crews been aware of the MCAS and there been a laid down procedure to deal with its possible faults then they might have only had unreliable airspeed to cope with, rather than fighting an unknown system which was trying to spear them into the ground.
Unreliable airspeed has been known about for years, crews are trained in the simulator and there is a procedure to follow in the QRH. The crew of the Malaysia Airlines A330 in Brisbane last year which experienced unreliable airspeed were able to get back on the ground safely.