Because there are only two manufacturers. It's impossible to get significant A321 delivered within the next decade as Airbus is (rightfully) sold out for most of their production.
Also it's extremely simpler for a 737 airline (the biggest being Southwest in the US and Ryan Air in the EU) to put another 737 in their arsenal rather than switch to a different manufacturer's plane. Anything from in-house maintenance to training to rostering to having the right type-rated pilots etc is easier for those cheaply run airlines to do when they're running just one type of aircraft.
Why airlines still order 737s at this point is beyond me. I guess some value having every type of aircraft in their arsenal for just-in-case one gets grounded while other airline groups are just caring about getting the cheapest thing (Lufthansa)
Also it's extremely simpler for a 737 airline (the biggest being Southwest in the US and Ryan Air in the EU) to put another 737 in their arsenal rather than switch to a different manufacturer's plane. Anything from in-house maintenance to training to rostering to having the right type-rated pilots etc is easier for those cheaply run airlines to do when they're running just one type of aircraft.
Why airlines still order 737s at this point is beyond me. I guess some value having every type of aircraft in their arsenal for just-in-case one gets grounded while other airline groups are just caring about getting the cheapest thing (Lufthansa)