The main driver is the desire to make engines bigger, thus more fuel-efficient. The fuel savings of the newest engine generations are significant. If you hang engines from your wings (I assume that has practical reasons, it is easier to hang the engine than include it into the wing, it is also easier to maintain), larger engines mean engines further from the axis. That applies to all modern airplanes which share this configuration. A complete redesign of the 737 wouldn't have made this go away either. However the goal of keeping the same type rating made the especial design of MCAS necessary.
Non-engineer here. What's wrong with taller landing gear to provide more clearance?
In 737 case, it means redesign of the landing gear system, fuselage modifications, recertification, retraining, new support system requirements eg. Higher stairs, cargo support equipment etc.
You also can't put longer main gear: as they fold inward, they have to attached at points separated by over twice the height. If you move them apart, you have to make the center fuselage as well as the root of the wings much stronger. That's a much bigger change than anything else.
I believe they altered the landing gear so it extends upon deployment, giving it a bit more height, but that's not enough. They can't just put longer legs, there isn't enough room. There's stuff there (cables, devices, bulkheads ...) and moving them requires a significant redesign. For the main gears, they would have to be attached further away from the center lest they bump into each other, which means the wings and fuselage would have to be stronger to accommodate for it. Very quickly you're just designing a new plane.
What about telescoping landing gear?
The thing is, this wouldn't have magically solved the issue. Larger engines hanging from your wing fundamentally affect the balance of an airplane. Take the thrust vector for example. The further you move the engine from the longitudinal axis of the plane, the stronger the angular momentum is, that is exercised from the engines onto the plane, especially when pitching up.
Taller landing gear makes loading the plane (especially with luggage) much harder.
Time, money.
They don't need to redesign the wings to accommodate the newer, larger engines.
There's only so much room for the landing gear. Larger diameter engines require longer landing gear. Increasing the space given to landing gear requires changes to the wing box. Changes to the wing box is fundamentally a wing redesign.
Which is why the A320 is designed to be loaded with conveyors. This means it has no problem accommodating an even larger nacelle than the 737 in a regular position instead of the up and forward the 737 Max resorted to. Even the previous generation 737 was running into this issue and that's why they went with the distinct oval nacelles.
iirc that's another term for larger, more efficient engines hanging lower from the same wings