The CFM Leap-1B (new engine on 737 Max) are bigger than the CFM56-7B, so Boeing had to change the mounting point of the engine (further forward and higher on the wings).
The gear is retracted inboard, so lengthening was a non starter given the rapidly escalating hardware changes that would be required to the gear bays.
Once you start changing the fundamental structure of the air raft, you also start weakening your case for a common type certificate as well.