For mangos the varieties that dominate in the US are the ones grown by the mexican and south american producers. They actively lobby to keep out other producer, and they can't just change what they grow overnight (nor does it benefit them since they already control the US market).

US food inspectors are required in places like India and Asia, but if enough inspectors aren't hired then you can't expect the exports to grow in a meaningful way. As you can imagine incumbent mango producers in the US don't want more inspectors in these countries. It's basically a cartel.