19 cities are served by a Rapid Transit with 12 distinct systems. Delhi Metro is bigger than London Underground. Do we still need that many Airports?
Additionally, like other have noted, transportation infrastructure comes into play as well.
I did also consider drug trade since that's one of the things, for better or worse, that comes to mind when considering aviation and the Americas. I guessed all the SA countries, which is the only reason I got Paraguay, I was surprised how many they had.
And, recently, drug trade.
But somehow got 100% first try lol