Heavy competition and if you were flying with the Taiwanese 3 BR長榮航空 JX星宇航空 CI中華航空 you can actually make this routing.
That's SEA-TPE-MNL(马尼拉)/CEB(宿霧)/CRK using the Taiwanese 3 CI BR JX. The first two are always timed to Manila Bound flight CI701/BR271 from CI021/BR025. Want CEB from TPE BR25/CI021/JX031 to BR281/CI705/JX781 which would make it a better alternative to PR124/125 then MNL-CEB on PR. Which the Taiwanese and Hong Konger spelled out as 菲律賓航空. If one does Delta Airlines Seattle to Manila the one leg would have to be on Philippine Airlines Taipei to Manila Terminal 1. Unless you are able to convince Delta Airlines to put you on the Skyteam partner CI for TPE-MNL. Alternatively you would be going KE042 to KE623/625/621 ICN-MNL if DL cannot send you via Taipei to reach NAIA from Seattle Tacoma International Airport.
Miss BR025/271 the alternative option they will offer you are on UA via SFO UA737 SEA-SFO then UA189 SFO-MNL. Followed by OZ SEA-MNL via ICN and lastly SQ27/918.