Because Tyranny of the Majority is a thing and it can be just as dangerous as Tyranny of the Executive. Tyranny of the majority is how lynch mobs occur (often times against the wrong person), why racial segregation laws lasted as long as they did, why it took so long to get gay marriage legal, why strange public pandering policies get nominated, etc. Just because the majority believes in something, doesn't make it right. Humans are very apt at group think and tribe think even if it is wrong, especially if they are closely connected. The fact that you must consider the small states serves as a sort of check on some aspects of tyranny of the majority.
It also serves as a means from trying to limit the stratification of wealth and influence within this country. By their very nature, the areas with the majority of the population already have the most influence and money. If you have to pander to the smaller areas, you can't just vote to concentrate ALL of the wealth and power in the most populated areas and ignore everything and everyone else. If you hate the current system and how wealth is being stratified, it will be dramatically WORSE in a system where not only do you have two parties, but those parties only answer to a chosen limited set of locations.
If you want a system that evolves and adapts with the times. You almost must have a system that respects the "little guy". Because any movement initially starts as a select few and changes things over time grow to become a dominant force. If the system only respects the majority, you will never see any true change as the little guy will always get "nipped in the bud".
There are very good reasons to implement this system the way it was done, even as imperfect as it is. Whether it is the BEST system to accomplish the best outcome is certainly debatable. But without a doubt transitioning to a majority rules system would have some VERY unintended consequences and possibly counterinuitively exacerbate some of the same things you are trying to do away with depending on your political leanings.
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