> it's not fully known yet why that procedure didn't prevent the accidents
The only way in the 737 Max to disable MCAS, is to disable electric trim altogether. That means only manual trim is available. When MCAS has trimmed all the way to one of the extremes (as happened in the Ethiopean flight), it requires many revolutions of the trim wheel and superhuman strength to return it to a more appropriate setting (see e.g. https://youtu.be/aoNOVlxJmow?t=750).
Previous generations of the 737 had two buttons related to the electric trim: one disabled the automatic trim system, the other one disabled electric trim altogether. The 737 Max still has two buttons, but they both have the same effect: disable the electric trim altogether. The old system could easily have prevented the crash by disabling MCAS but leaving the electrically assisted trim enabled.
Boeing's procedure to make manual trim adjustments possible is to put the airplane in a dive, because that lowers the aerodynamic forces on the trim surface. When you're only a few hundred feet above the ground, that's not really an option. The pilots re-enabled the electric trim in the hope they could use it in their fight against MCAS. It's the only thing they could think of in the stress of the moment, faced with near-certain death and an airplane actively trying to crash.
In hindsight, after reading accident reports and thinking about it in the comfort of chair that's not crashing, there were things they could have done. Lowering the throttle would have lessened the forces on the trim surface, activating the flaps once the speed was low enough would have disabled MCAS. I can't blame the pilots for not coming up with that in the heat of the moment.