Because there’s a limited budget for internationals (separate from Americans’), need-aware/meet need elite colleges* want to have the best way to allocate funds and for this they need to have an overview of all internationals applying. That’s why they prefer RD or may offer EA applicants a financial aid package later on. To be blunt, if you have $200,000 for your international incoming class’ financial aid, you can’t afford to blow $100,000 on two top students in the ED round when you could have 3 equally top students in the RD round use that amount. However since top colleges are competing with the likes of Harvard for the internationally-top students, they’ve relented their policies.
(BTW internationals are encouraged to apply earlier than the deadline.)
Up until the 2016 cycle both Carleton and Grinnell explicitely requested that international students anticipating significant financial need apply in the RD round only. Traces of this have been erased from Grinnell’s website but can still be found on Carleton’s
- The situation isn't the same for need-blind/meet need colleges, of which there are 5: Harvard, Yale, Princeton, MIT, and Amherst. The budget there is not limited in the same way so that they can admit as many super needy applicants through the early round as they wish and be pretty certain these financial aid packages will be unmatched by other universities.