According to https://sportstoursinternational.co.uk/blog/how-to-get-entry..., you can also gain entry via a tour operator for non-USA entrants or if you've already run several of the Abbott World Marathon Majors, there's a draw program.

Boston Marathon tour operators: https://www.baa.org/races/boston-marathon/plan/international... and https://www.baa.org/boston-marathon-international-tour-progr...

Abbott World Marathon Majors draw program: https://www.worldmarathonmajors.com/content-hub/majors-draw-...