The 737 airframe reached a point where it was not compatible anymore with modern engines. When the 737 was designed, engines where a lot smaller, the nwer turbofans simply don't fit under the wings of a 737 anymore, so they had to be moved forward. That changed flight characteristics, Boeing used MCAS to compensate for that. If I remember correctly, on-board systems of 737 had issues with handling a second sensor for MCAS (someone with more knowledge please skim in). So they went with that config, they went, as we see in the messages from the chief tech pilot, to forgo major re-certification and thus decided to hide MCAS true nature and impact from the FAA. Consequently, they also hid it from EASA since FAA and EASA basically trusted each others certifications.

Boeing, if you ask me, committed a cardinal sin in aerospace. They cut corners, ignored redundancy, lied to regulators and as a result directly caused airframe losses killing crew and passengers. And that after decades of efforts to increase safety. All that just to save money and maybe keep market share.

Had they just done all the proper testing and development they did after the aircraft losses upfront none of that would have happened.