> 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.