I believe that it's mostly a question of incentives.
IMO/IOI are very prestigious competitions for high school students. Doing well there can help you get into the college of your choice. This is true for US students as well as for students in other countries.
ICPC is a college students competition, but if you are a student in a good US college there isn't a lot to gain from it. You have plenty of real world opportunities already: tech industry, startups, academic research. In contrast, if you are a student in Russia or China and your goal is to get into a US Ph.D. program or to work in a place like Google or Facebook, ICPC is a great opportunity to make yourself stand out.
Keep in mind that a large fraction of the top IMO/IOI students from around the world move to the US for college, so there's plenty of talent in the top US colleges. There is just no motivation.
One other factor is support from the university. Places like MIT/Stanford don't care much about ICPC. The coaches are usually other students that are former contestants, working on a volunteer basis. On the other hand, a place like St.Petersburg ITMO cares a lot about ICPC, because it gives them bragging rights.
(Source: I've studied both abroad and in the US and I've been a contestant.)
IMO/IOI are very prestigious competitions for high school students. Doing well there can help you get into the college of your choice. This is true for US students as well as for students in other countries.
ICPC is a college students competition, but if you are a student in a good US college there isn't a lot to gain from it. You have plenty of real world opportunities already: tech industry, startups, academic research. In contrast, if you are a student in Russia or China and your goal is to get into a US Ph.D. program or to work in a place like Google or Facebook, ICPC is a great opportunity to make yourself stand out.
Keep in mind that a large fraction of the top IMO/IOI students from around the world move to the US for college, so there's plenty of talent in the top US colleges. There is just no motivation.
One other factor is support from the university. Places like MIT/Stanford don't care much about ICPC. The coaches are usually other students that are former contestants, working on a volunteer basis. On the other hand, a place like St.Petersburg ITMO cares a lot about ICPC, because it gives them bragging rights.
(Source: I've studied both abroad and in the US and I've been a contestant.)