If the decision is too fundamental to be made by the people, then we're saying that precisely the most important matters should be decided by people with interests that may not be those of the people whose interests we're actually trying to promote, which is the general public.

One of the main ideas behind this type of Constitutional interpretation is that these decisions were already made by the people. That's what the Constitution is, and it's why states and Congress can't pass certain laws, because they conflict with what the people have decided in the Constitution.