I blame it on the high bypass turbofans: The '73 was designed for straight pipes and has been fudged (quite successfully) since the Classic series. But these LEAP engines gave Airbus a HUGE advantage because there was room under the 320 wings for 'em. Those big-ass fans. The "-1B" engines themselves are a compromise vs the "-1A" -- (9:1 vs 11:1 bypass ratio).

The icing on the cake was the constraint that the MAX must be common type -- at all cost: clean sheet was not an option because of development time and another $10B or so in costs.

The huge enabler? The FAA delegating verification of certification requirements to Boeing. The hen guarding the foxhouse.