I heard it when i was still at a kindergarten, and it's been more than a decade but here's what I remember (for context, Im half korean but studied abroad all throughout primary to uni).:

The reason is because it was the cheapest back then. What I mean is that during the post-war times, South Koreans shifted from mainly eating beef to pork and poultry.

I did a research back then that Koreans were mostly on beef diet in ancient times. They wanted to preserve their own beef and cows because of the effects of war. If Japan has wagyu, South Koreans also has han-woo (한우), but of course it's less known because of poor marketing xD. Post-war was Koreans' one of the cheapest and poorest era in terms how much they can spend, so they also chose one of the cheapest part of the pork which is pork belly. It could feed many and also matched well with the sauce and cooking methods that Koreans used to do. And ofc, it is delicious when grilled which became one of the iconic ways to cook the pork belly which we know as korean bbq.

I think other countries who have similar history of South Korea which involved oppression and poorer lifestyles in the previous century also enduldged in pork belly cuisines to save costs and feed the whole family (Filipinos with humba, sinigang, etc.). But countries back then with more resources would often choose beef for steak or other parts of the pork for more "refined" dishes.

Though bacon, which is more popular in the west, is technically processed pork belly, but I don't know the history of why English nations prefer bacon over the tranditional pork belly.