There was a specific part of the flight envelope where a higher angle of attack would lead to less pressure on the stick. This was a result of the engines being

There was a specific part of the flight envelope where a higher angle of attack would lead to less pressure on the stick. This was a result of the engines being further forward, and thus at high angles of attack having more 'leverage' trying to pitch up the plane. This had two problems. Firstly, this flight characteristic (more AoA gives less stick pressure) is considered risky, because it can trick pilots into thinking the AoA is decreasing. Secondly, and apparently more importantly to Boeing and American Airlines, this meant that pilots certified to fly the old 737 would need to go through new training. This new training was required because of a significant change in flight characteristics and would be very expensive to airlines. So Boeing created MCAS, and tried their best to hide the change so no one would question whether new training might be required. If I recall correctly lion air (the airline of the second MAX crash) had actually asked Boeing if they should do some new simulator training for the 737 max in an e-mail and had been ridiculed for even asking that.