MCAS was a bad sticking plaster on a different problem - that the 737-800 doesn't have. To stick larger engines on the 737, they had to be moved forward. This

MCAS was a bad sticking plaster on a different problem - that the 737-800 doesn't have. To stick larger engines on the 737, they had to be moved forward. This meant that if the nose pitched up, there was a large area ahead of the wings "catching wind" and pitching the nose further. The MCAS was a "solution" to that - if it detected the nose pitching up, it would counter that by trimming down. In the MCAS-related crashes, the sensors malfunctioned and airplane just pitched down until it crashed.