I have read smatterings from Filipiniana books. That means, I am not that well-verses into the topic. However, as I ponder upon what I remember, I came to this conclusion:
The colonizers and then the elite want a united government.
I can understand why America & Spain want only one government in the island. They wanted simpler governance, anyways, from the mess that was the Negros Republic (is it? I forgot, it's somewhere in Visayas), revolution, Sulu, & the ecomienda system, respectively. That simpler set-up presented by the Americans allowed the elite to have easier time setting up for their own purposes, that is by building up their own machineries, & patronages, sending representatives to the legislature, getting favours from the politicians that was, easier business, etc.
After all, seperate states on both sides of Pasig River, let alone between Bulacan and Pampanga, had been a political insanity since the Spaniards consolidated their rule here.
Now, for the popular side, aside from not having much bad blood between most of the ethnic groups (except maybe with the Moro & Christian groups, or Tagalog-Pampanga gap), having quite easy transport by sea, and by land (easier in the gay days with PNR, we're just patient with bises nowadays). So, you could say, "Proud Bicolano", or "Bisaya ka?" without causing much ruckus. The numerous Muslims here in the barangay are not offended that much when an evangelical suddenly starts to preach in the street. Of course, stereotypes about each other (Manila snob is the most obnoxious) are a thing, but we tend to laugh it off. We don't like to escalate things.