Lot of discussion on MCAS and associated issues, I'm not condoning Boeing's apparent quick fix but airlines and consumers have a degree of involvement too.
737 is by today's standards an old design, the NG was a comprehensive update but its still based on a 60's design.
If you look at the 757 which most of us thought would take over from the 737 it takes a greater payload further, higher and faster for a comparable or better fuel burn; Not seen much outside the States now but still heavily used by some major players for N. American routes.
Why choose an older design over newer, more efficient types? Conservatism and customer familiarity/confidence are my first thoughts, leading us to this moment.