Primarily because the original states were sovereign before creating the union, and they formed a republic that maintains some level of autonomy for states, as

Primarily because the original states were sovereign before creating the union, and they formed a republic that maintains some level of autonomy for states, as the people in those states wouldn't want to loose their sovereignty completely. California and Texas joined because the benefits of the larger union outweighed staying independent (no doubt Texas second guesses that at moments). Fast forward a hundred odd years, and the system while somewhat unfair on a per count vote still at the federal level still allows for greater overall autonomy for citizens of different states. California can and does use its clout and local autonomy to make laws that its population wants (e.g. car emissions standards) without forcing the same on the rest without more effort. It's also why the increase in federal power and presidentialism in both major parties leads to less overall individual autonomy and control, IMHO. Even with a direct 1-1 vote count, a single vote for a president out of a 100+ million votes has much less effect than the same vote for a governor in a state with a population of even tens of millions of people. Generally, the federal government should only make laws that need to or can only be made nationally. The rest should be state level. Its less efficient but generally considered more robust. Also, to a degree the idea of a republic of states implies that the 4 million people of LA cant readily "group-think" the rest of the people in the union into whatever laws enable or inhibit whatever lifestyle/beliefs/etc they want via the federal government. In that sense it maximizes the fairness of "tribes of people" slightly over that of single individuals, which seems consistent for a republic rather than a more direct democracy.