Why is Metro Manila considered a primate city?

Metro Manila in the Philippines is a city full of attractions for the tourist, as well as being the seat of government, and therefore primate. It is also the title of a film, 2013, I believe. Its official name is National Capital Region, but Metro Manila is its famous name and the one in most use.

It is one of three defined metropolitan areas in the Philippines. Metro Manila is rich culturally, having eight or more museums, art galleries, wildlife tours, nightlife, and everything to make a person's stay there thoroughly exciting and enjoyable.