you will not know the criteria for sure.
if you really want that program badly, then email the affiliate manager listed in the program detail page.
if you are not within US, many will be declined by default, but that does not means it's not possible to get approve.
i have a US financial program which decline me by default, then i wrote to the affiliate manager.
what she really wants is that i make sure that my traffic is for the US and I have a website promoting the program.
once you have that, then you are likely to be approve.
but that said, some will decline you no matter what, if you are not in US