Student loans in the U.S. are guaranteed by the government and you can’t charge them off - even if you declare bankruptcy. This means that student loans for U.S. citizens will be repaid even if someone defaults. Because of this, lenders are willing to provide lower rates to U.S. citizens who want to refinance their loans.
This rule cannot be enforced on International students because they can leave the country any time if they can’t find a job or don’t get the right type of visa. Student loans are also generally larger amounts and are paid back over many years. It is impossible to predict if an international student will be able to stay in the U.S. for the term of the repayment period.