Why is manila considered a low-yield destination for airliners?

Direct flights are expensive and the number of pengers going straight to Manila and out wont always fill a plane (save for Christmas season). Manila has not yet captured the interest of as many folks who would go Bangkok, Hongkong or Singapore. Add to that, most tourists/visitors would prefer to also hop into other nearby Asian cities you mentioned. PLUS, there are a lot of cheap flights to/from Manila to these Asian cities. Example: a backpacker from London thinking of visiting Sagada would not let go of the chance ping by Bangkok or Singapore before or after :)