Fewer US are probably applying to cards because of its crappy lifestyle. It makes a bunch of money but definitely has one of the worst lifestyles. The top US applicants these days are applying to the residencies/fellowships that have better lifestyles (surgical subspecialties).
As for international applicants, they are probably just as smart and "top" as the US med students/residents, but coming from a foreign med school basically excludes them from the ultra competitive residencies. The moderately competitive ones (i.e. internal medicine) are the best residencies they can get into it. It's technically not fair for them, but US students would argue it is fair and say US residencies should give preference to US students. So anyways, these international students are just as naturally competitive and smart as the rest of us, so they want to get as high up as possible. Generally, the best residency they can get into is internal medicine, and the "best," most competitive field under that is cards. That's why you see so many cardiologists who went to foreign med schools. These FMG/IMGs in cards are very smart, hard-working, and excellent docs, arguably even better than US ones since they had to work so hard to get there considering the stigma they faced in the US medical education system.
