AoA sensors
Until MCAS was introduced the AoA sensors only provided a warning indication to pilots that they were approaching a stall. If the AoA sensor failed it could be ignored by the pilots who could fly the plane perfectly well without it.
MCAS as implemented on the 737MAX uses inputs from one of the two AoA sensors to drive control surfaces without pilot input if the sensor reports an approach to stall condition. If the sensor fails and reports an approach to stall in certain circumstances MCAS will pitch the plane's nose down forcefully even if it's flying level or in a climb.
Fitting an extra AoA sensor would require a lot of recertification paperwork, engineering changes etc. After that the MCAS system would need to be restructured and rewritten to deal with two-out-of-three voting and even then it still might fail on occasion. Doing all this would have indicated to the licencing authorities that something was wrong and it's possible the 737MAX would have required certification as a new aircraft type. Preventing this certification change from happening was the entire point of MCAS in the first place.
Biting the hand that feeds IT
