I actually think that this is the major reason, not that competition changes.
Having taken a fair amount of actual medical school coursework, I have been surprised by the amount of knowledge of physics, organic chem, bio, and gen. chem that is needed at a basic level in order to be up to speed in medical school. I have tutored students in the medical school biochemistry, and if someone's background is not strong in chemistry or mathematics, for instance, picking up enzyme kinetics or stoichiometry is not easy.
Same goes with physics/bio and physiology, in which you study fluid flow, pressure/volume relationships, electricity and current, etc.
People need to have the coursework somewhat recent to keep from lagging behind the rest of the students in classes where extra time spent for background is just not feasible.