Could be because of the serious repercussions to the markets if the information that it was a bombing became public. It's easier to lay blame on faulty waste storage/disposal than to reveal that it was actually military grade explosives which blew up Glorietta.

Strangely, even the PNP's own forensic consultant (I forget her name) who specializes in investigating bombing incidents said that it was a bomb explosion and not methane gas as claimed by the PNP.