philippines wasnt labeled the 'pearl of asia' for no reason. it is economically the center of asia. it is probably the busiest trading ground of all asia. this makes an ideal 'center ground' for the ufc. yes, japan probably have the most mma fighters (korea getting up there too), but japan is culturally hated by most south east asian countries (goes back to WW2). so philippines can actually reach more countries (such as malaysia, indonesia, thailand, who are all actively competing in muay thai) than japan could. while the quality of those fighters might be questionable, low production/advertising cost combined with the reasons mentioned above make the philippines the ideal starting ground for asian media market.
and philippines has it's name to hosting big events, no combat sport fan doesnt know about muhammad ali's "Thrilla in Manila" fight with joe frazier (which is considered one of the best boxing fights ever). and of course, it's home to the pound per pound boxing sensation manny pacquiao.