It's not that it's more efficient, it's that it doesn't require much training for the pilots who were certified on the older 737's.
Basically it saved a fair bit in training money for the airlines, as moving to a different airframe requires specific training, as does moving to newer versions of the same airframe which are materially different in the controls or handling, by using the loopholes in the regs Boeing managed to get the FAA to sign off on pilots moving to the new version with just a few hours of classroom familiarisation (IIRC one of the deals boeing signed offered up to a million per pilot if additional training was required).