True, but CUN has far more connecting opportunities out of SEA that is not too inconvenient like DFW, SLC, ATL, etc... If you want to fly to SJO you will have to connect in DFW, MIA, or ATL... None of those connections makes sense..
Believe it or not, SEA-LAX-SJO is LONGER than SEA-DFW-SJO (thanks to the curvature of the earth) and of all the connecting points (PHX on US, DFW on AA, DEN on F9),
would be the longest route.
I also find it pretty interesting that the Bay Area has no direct SJO service (you'd need to fly a UA codeshare down from SFO)- a much LARGER market than SEA, and this indicates to me that the demand you'd be able to get from SEA for LAX-SJO is pretty piddly. You'd really be needing demand from LAX to make this work, and I am not convinced it's there.