I was born in the Philippines and they LOVE Spam over there.
I think it does have something to do with WWII. Things that the military ate became treats to the citizens because they would be given them as gifts or rewards, or as rations. Anything American was very highly esteemed. Up to this day Filipinos love processed cheese and canned corned beef too. My grandfather worked underground for the Americans in some kind of radio operator capacity (we never knew exactly what he did - he died about 5 years after the end of WWII and he never talked about the details with his family). My mom remembers him bringing home Spam and cheese every once in a while.
My father was in Leyte during WWII. He never saw combat. He said his job was to guard the chow lines to keep starving Japanese soldiers, masquerading as Americans, from sneaking a meal.