Designing longer landing gear might invalidate the common type rating, as the plane's entire fuselage and wings would be higher off the ground during take-off and landing, which might be non-trivial. Plus it would have different heights compared to previous 737s, which could cause problems with jetways, staircars, maintenance vehicles, luggage ramps, etc.
There's not many things they could've changed about the existing design without it being enough unlike the existing design to effectively count as a new aircraft (logistically if not certification-wise). They could move the engines, but they couldn't change the ground clearance.
The MAX-10 has longer landing gear by 10 inches and I'm assuming it shares the same type rating as the rest of the 737 line. So it is possible.
However the MAX-10 required a telescoping landing gear to achieve that so I'm assuming the real reason to not do so earlier was cost to design, cost to manufacture and weight.
I think the MAX 10 landing gear changes allow for a higher rotation angle (without tailstrike) on takeoff/landing, not actually increasing the engine-ground clearance.
Also the MAX 10 hasn’t been built or certified yet, I’m sure there will be a lot more scrutiny when that does happen.
There's not many things they could've changed about the existing design without it being enough unlike the existing design to effectively count as a new aircraft (logistically if not certification-wise). They could move the engines, but they couldn't change the ground clearance.