I think it's the combination of integrating the battery pack into the lower frame structure of the vehicle, and also integrating all of the drivetrain component

I think it's the combination of integrating the battery pack into the lower frame structure of the vehicle, and also integrating all of the drivetrain components into that flat frame. In a body on frame vehicle the engine still sticks way up above the frame and has to be accounted for in the body design. Everything built on the F-150 frame looks like an F-150 (F-150, Expedition, Navigator), but something like VW's MEB platform can have a lot more vehicle shape variety (ID 3, ID Crozz, ID Buzz, etc.)