Social and political interests tend to have a heavy coincidence with geography. If you are on the coasts you care way more about the fishing industry than people in the heartland. If you are in a desert you care more about water conservation. If you are near oil and natural gas your livelihood or the livelihood of your neighbors probably depends on the energy industry. By virtue of being in a population dense area, you automatically have a powerful voting block on various area specific issues. What's more, the people in other areas are not your neighbors, you have much less incentive to protect their interests, and are much less likely to hear their anger and complaints when you don't. By and large people from Wisconsin are not going to be able to come and protest march down the streets of LA if California -- 8 x the population of Wisconsin -- decides corn should be taxed to subsidize making action movies.
The senate already gives proportional control by statehood and prevents that sort of issue.