A key to understanding this is "type rating". Pilots are generally certified to fly 1-2 types of aircraft at a time. Budget airlines, in the US in particular, w

A key to understanding this is "type rating". Pilots are generally certified to fly 1-2 types of aircraft at a time. Budget airlines, in the US in particular, will try to maintain a single type rating across their entire fleet. This means planes are largely interchangeable with some exceptions (eg a given plane has to be certified to fly over open water on routes like Hawaii and to Europe). So any pilot can fly any plane in the fleet. That's great for keeping costs low and scheduling. As soon as you add a new type rating, you've created a second pool of planes and pilots that you need to manage. Airbus surprised Boeing when it introduced the A320neo as this was very attractive to budget airlines who were loyal customers to the 737 over decades. There were multiple iterations of 737 up to the 737NG (to that point). Beoing had a choice: develop a new aircraft. This would take longer and wouldn't keep the same type rating so they'd lose their type rating lock-in with the likes of Southwest, who is a huge Boeing customer. The compromise was the 737Max. It was faster to develop and deliberately designed to maintain the 737 type rating. But it had larger engines and they were moved forward on the plane. This could make stalls easier so Boeing (mostly) silently added a system to "handle" that. That was the infamous MCAS. Lack of knowledge about this and training was what caused the 2 crashes that grounded the Max fleet last time. This was exacerbated by the sensor having a single point of failure. Interestingly, there was an optional upgrade package to have 3 sensors instead of 1, which is more the industry norm for critical systems. 2 of the 3 sensors have to agree. So Being solved a short term problem but created a long term problem as that 737 lock in was both a blessing and a curse. Existing customers liked the 737 but it was an aging plane that didn't fit changing standards with plane controls, engine design and so forth. Also, to keep costs low, Boeing has increasingly over the last two decades relied on various layers of outsourcing. Airbus simply isn't shackled with 50 year old designs like Boeing is. They've made mistakes too, most notably the A380 but between the A319/320/321 (neo and not), A330 and A350, Airbus has a really solid product line. Boeing's 777 was a workhorse for long range wide body aircraft in the same market as the A350. The 787 fits here too but it seems like it's not as popular as the 777 or A350. I could be wrong. Lastly, Boeing's 747 was a massive cash cow for decades. This is what let them own the airplane market. But 4 engine planes no longer make economic sense. The same applies to the A380. So we have the 777/787/A350 instead.