This is a great discussion.
Speaking as an American who worked in Manila for two years, I'd say the main reason Phils is far behind is because the rest of the tourism industry is stunted. Unlike, when I worked in China, friends and family from home had no desire to visit me in the Phils.
I mean, no offense, but Thailand and Indonesia offer Americans a much more "exotic" cultural experience.
One thing Philippines should do is embrace its WWII history. It should follow the model of Europe and make outdoor museums of battle fields (or like what is done in the US South for the Civil War). It's a shame that so much of that important history seems to have been forgotten. The Phils and brave Filipinos played such a critical role in WWII. Americans would be fascinated by it.
If something like that was embraced, it would be a big plus as a non-diving activity to attract Americans. The only place you can really find it is Subic and that's only because it was the old Navy base.
Further, other non-diving activities like guided mountain treks or rafting are tough to find.
Hey, Philippines has got boxing, jai lai, and cockfights, which are great, but it needs more.
Thailand, Bali and Malaysia all have exotic temples, and exotic culture that make them more enticing.
Philippines needs something like that.
Finally, Manila is on the same economic-poverty level as Lagos, Nigeria. Its a turn off.
This year, my big dive trip is to Indonesia. (At least Phils has safer Airlines than Indonesia)
Just some thoughts...