The 737 has gone through many design iterations.  With the introduction of the Max, the rear fuselage got quite a redesign.  The original 737 tailplane was redesigned for the NG family when it was widened.  This affected the rear fuselage a small amount.  However, there were clearly still issues with that part of the plane given the large number of vortex generators that were fitted.  These are always there to fix things that aren’t quite as they should be.

The Max has a totally reconfigured rear fuselage.  Boeing clearly decided to clean up the issues that they had been living with.  Any opportunity to reduce drag is needed when you are competing with the newest version of your opponent.  They have thinned the fuselage a lot and this is one of the easiest ways to spot that a jet is a Max rather than a previous version (the others being the engine size and the winglet type).  The APU inlet has also been relocated.