I think it has to do with cost savings (although they of course market it as lowering the "environmental footprint of the series" and other gobbledygook excuses) . The latest trend is to go racing on what looks to the outside world as a "shoe string budget". It's very PC and is supposed to guarantee sustainability. They (the regulators) are trying to break free from the boom and bust cycle that has been a feature of racing for decade upon decade. The result is this very watered down type of racing , that to old goats like me, is as about as interesting as watching paint dry.