If you look at the United States as a collection of State governments, it makes perfect sense. I'm surprised you are confounded by that logic.

It was actually the most controversial issue during the Constitutional Convention (See "The Great Compromise"). And read up on the Federalist vs. Anti-Federalist debates.

One of the main points is to not give all federal power to the top 9-10 states. Otherwise you end up with the situation where a few large cities are deciding things at the national level and there is no representation for rural areas.

As an extremely over-simplified example, imagine urban voters voting to outlaw tractors because "Tractors emit huge amounts pollution and nobody I know even uses a tractor! Those dirty farmers that I hear use them are dirtying up our air!"