I´ve been wondering a little bit about the same thing. Why don´t they want to promote tennis in South America? Buenos Aires, Argentina or São Paulo, Brazil could be good candidates for the YEC. Or maybe Latin America would need a tennis version of Shakira do it possible or someone like Gabriella Sabatini was at her best.
This shows how much WTA & ATP care about globally promoting the tour, and tennis in general, even though they say that's a reason to give events to Asia. But the main reason for WTA/ATP, as well as e.g. Formula 1, to give events to Asia is that they get most $pon$or mon€y from there. Far East and Middle East are the areas where they pay most for hosting a tournament. Latin America, on the other hand, is mostly unable to pay as much, and thus they get only MMs in both WTA and ATP Tours. And I don't think a successful WTA player would enable big tournaments in Latin America, Argentina has many good ATP players and still they have only an ATP 250.
But, luckily for Latin America, Brazil is one of those rapidly developing BRIC countries, and there seems to be interest in getting top-level tennis to their country. WTA will have already this year an International event in Florianopolis (BRA). And next year there will be a joint ATP 500/WTA International in Rio. Plus, Rio seems like the most likely venue for ATP WTF after London's contract expires; Rio de Janeiro is already a sponsor of ATP WTF. And if WTF were a success in Brazil, I believe they'll get an ATP Masters, just like Shanghai got after WTF.
Of course, that's only ATP. For some reasons, Latin America has many good ATP players but I can't remember any Latin American WTA player. So men's tennis is clearly dominant in Latin America, and they also have more ATP than WTA events. So even if ATP had a Masters in Latin America, I wouldn't be too surprised to see WTA not having a proper Latin American swing. But new WTA events in Florianopolis and Rio seem promising for WTA, I believe that in 2020s there will be at least an ATP Masters and WTA Premier in Latin America, most likely in Brazil.
But considering Brazil's obvious desire for big tennis events, YEC would've been a good chance for WTA to market themselves in Brazil before ATP comes with something big.