Accepted by European university, rejected by all American ones I applied to? Possible reasons?
It is sometimes difficult for admissions committees to evaluate applicants coming from universities or university systems that they don't have direct experience with.  This is especially true if they also don't know your letter writers.  It can help to apply to universities where someone in the dept. has some connection to the country you're applying from.  Since someone at Frankfurt knew one of your letter writers, that person may be better informed about your letter writers, the quality of your school, and the meaning of your grades.