tl;dr: Autonomous driving uses a whole host of multiple and different kinds of sensors. Musk said "NO, WE WILL ONLY USE VISION CAMERA SENSORS." And that doesn't work.
Guess what? I have eyes; I can see. You know what I want an autonomous vehicle to be able to do? Receive sensory input that I can't.
We also use way more than just our eyes to navigate. We have accelerometers (ear canals), pressure sensors (touch), Doppler sensors (ears) to augment how we get around. It was a fools errand to try and figure everything out just with cameras.
What’s worse is it will be hard to reverse this decision. Tesla is a data and AI company compiling vision and driving data from drivers around the world. If you change the sensor format or layout dramatically, all the old data and all the new data becomes hard to hybridize. You basically start from scratch at least for the new sensors, and you fail to deliver a promise to old customers.