Residency comes down to risk. Which applicant is more risky than another?
Is someone clinically competent? USMLE, other than being a test of test taking skills, is suppose to be a standardized clinical compentency exam. But does a test tell the entire story in regards to clinical competency? No. Someone with stronger clinical rotations has a huge advantage over someone who does not. DOs and IMG/FMGs are at a disadvantage on the whole in this regard.
There are also questions regarding the type of person who enters an IMG situation. After all, who would enter a scenario in which you would leave the country to achieve a sub-par education, for just as much money, and with SIGNIFICANT higher rate of attrition? You would look at it one of two ways. The first way: "Wow, IMGs are such go-getters. They are willing to do everything possible to be part of this profession." The second way, "What a stupid decision. Why in the world would they do that? Could they not delay gratification for one more year to improve their scores? Did they have a significant, perhaps personality, red-flag during the application process that would make them so desperate to invest in such a horrible investment?" Unfortunately for IMGs, the second line of thinking is more prominent.
Third, PDs want to take a know commodity. If they have had nothing but USMDs, the USMDs have done fantastic in the past, they why would they pass on a known commodity for a DO or IMG/FMG? You take the known versus the unknown with a good test score. Also, in regards to known versus unknown...DOs live in the states. They have the ability to audition. About half of my med school class matched ACGME, and they did so by auditioning and doing well. If someone does well on an audition, it reduces the applicants risk level. It is difficult for IMGs to audition at most sites beside IMG-slave factories.
There are SERIOUS questions about the quality of education at IMG programs, the personality type of IMGs, and the reduced opportunity for audition. It's why IMGs are in the lowest rung of medical applicants...frankly, why applicants would be stupid to go that route in today's medical landscape...and why the military won't even allow them.