Not surprised that Belo has more than Rio.
Shouldnt Mexico city have a direct flight, based on those numbers?
Dividing by 365 and then by 2 gets you a PDEW (passengers departing each way) of 62. With the exception of Cancun, other Mexican destinations do not have a lot of passengers to/from Boston. United's Houston hub offers pretty much everything Mexico City offers.
I flew Boston-Mexico City-Acapulco-Mexico City- Boston when the non-stop was available in mid 2000's. I'm sure the weekend flights were full since many people on the flight were connecting to Pacific resort areas. Connecting at MEX was a nightmare though and word of mouth had to have spread quickly on that.