The location allows for contamination of the most fertile farmlands in the country, pollution Westward via the Mississippi River basin, and pollution East and South by wind. I can't think of a better location if your goal is to impact as much of the population as possible.

As for evidence, we have no evidence that the vaxx is a bioweapon and not just some colossal fuckup by greedy pharmaceutical giants, but we still think it because that's where the logic leads.