Not an aero expert, but, common sense suggests the cars running aggressive rake and heavy undercut tend to produce decent mid section down force (around the side-pod area). This might let them get away with shedding some front wing down-force without unsettling the car drastically.
One could speculate that Mercedes, due to it long wheelbase and more flat flow of the floor surface area, relies on balancing the down-force more evenly across the whole car. In other words, shedding front-end down-force might produce more lift in W10 than it would on SF90.
** purely based on my over-simplified/non-technical understanding (or interpretation)
*** I don't mean to say Merc's solution is wrong or Ferrari's solution is better, that remains to be seen. I'm just stating why a solution that works on one car may not work on another.