TL;DR -

Boeing needed a new plane but they wanted an old regulatory system. So instead of calling it the 797 or whatever, they called it the 737 MAX. And the 737 MAX didn't fly like a 737 and their test pilots said "put winglets on it" and Boeing said "too expensive/won't work" but since the places it wasn't flying quite right were corner-case they came up with an emergency fail-safe system that likely would never be used.

So then they flew it a bit and it didn't fly quite right so they bolstered out their emergency fail-safe system so that it wasn't really failsafe anymore, it was more augmentation.

And then to simplify it to get it through regulation, they got rid of all the sensors except one.

The current and former employees, many of whom spoke on the condition of anonymity because of the continuing investigations, said that after the first crash, they were stunned to discover MCAS relied on a single sensor.

“That’s nuts,” said an engineer who helped design MCAS.

“I’m shocked,” said a safety analyst who scrutinized it.

“To me, it seems like somebody didn’t understand what they were doing,” said an engineer who assessed the system’s sensors.

And that is how a system that was designed to never be used and relied on three different kinds of sensors became a system that was essential to flight and didn't even have a backup.