Corporate inertia. There wasn't clear ownership within FB to manage these kinds of political minefields back then. You have to remember that this was the 2010-2015 timeframe and social media was riding high due to the impact it had during the Arab Spring only a couple years prior.
> why is that content so popular with Burmese people
Because society in Myanmar is extremely communalized, with both ethnic strife among the 70 different ethnic groups and also religious strife between the 4 main religions.
Myanmar has been in a state of civil war since independence, and this is further exacerbated by regional powers fueling the flames by supporting militias along with the army.