Echoing what
@wondervisions said there's a pretty big difference between the career of a physician and the career of a physician scientist. Typically physician scientists are expected to spend 70-80% of their time doing research. Add that to the fact that it takes longer and most people don't want to do it.
I'd also contest the fact that the admissions are easier. In fact the GPA and MCAT are slightly higher for MD/PhD admissions (source:
MD vs
MD/PhD) than for MD-only matriculants. Throw in the fact that most matriculants have 2000-4000 hours of research you have a pretty competitive process. Usually a good MD-only application isn't a great MD/PhD application and vice versa because they focus on such different things.