How can something that important be an option?
Based on other comments here and in other discussions, it seems like adding that feature would increase the training requirements for the pilots since it is an additional instrument in the cockpit. I don't know whether it was Boeing, Boeing's customers, or both who pushed for the instrument to be an option. Any combination is plausible, and I'm not going to rush to judgement like a couple of my sibling commenters just because it's the fun and popular thing to do.
Basically everything about this plane boils down to "it should require more training but it didn't"
This seems like an important factor. I wonder if that's the reasoning behind the FAA not grounding the planes and assuming most US airlines have this system in place.
Yes. And the recorders have angle of attack data. We'll know within days if it's another AOA sensor problem.
Southwest, the biggest US operator of 737 MAX aircraft, bought the "AOA Disagree" option on their aircraft, and recently added an AOA indicator. Many operators didn't get that option; Lion Air didn't have it. If you have that, you get a warning that the flight control system is seeing bogus AOA data. It is somewhat less of a safety issue if you have the backup systems.
Why this aircraft was ever offered without "AOA Disagree" sensing is unclear. The single active AOA sensor can force the nose down. The pilots can switch to the other flight control computer and use the other AOA sensor, but at that point they're losing control of the aircraft.