Kansas City has one or two flights a day to Toronto and one or two flights a day to Mexico. Indianapolis has flights to Canada. Technically speaking they are "international airports" but I'm pretty sure that Amazon is looking for more international options than that.
St Louis has Lambert (or Lambeau as a a former president put it) International. And since TWA was absorbed by American which ended up abandoning it as a hub I would guess it has a lot of underused capacity.
There is a lot of abandoned, dilapidated property, unfortunately, it is not in the part of town that workers would feel safe commuting to. Lots of blacks in St Louis are holding down decent jobs and would be glad to get a solid job with Amazon—they are not the ones that are causing St Louis’s woes. In fact, if you look at the demonstrations you’ll see a lot of white Auntie Fas participating.
Most whites in St Louis city and county do not live in black-majority neighborhoods. That is seen by some as obvious racism.