..because English is a national language.
National newspapers are published in it; national TV programmes use it, it is the language of the Law and of Congress.
There are people who refuse to use Taglish because it is a language that can only be understood if you are bilingual (and "hip"!) in both English and Taglog so you don't "get through" to anyone else, and there are people who use English because their native tongue is not Tagalog and they don't see why they should use Tagalog if they are native speakers of Ilocano, Bisayan, etc...