Simple really

1) The base ingredients of a fried egg, sambal, chicken and a slice of cucumber, most importantly the chicken cooked in a method that we see as perfect. McDonalds being Halal also helped.
2) Nationalistic pride during its release made us Singaporeans want to try it, then some bloke says its good and by the power of social media & nationalistic capitalism, we all flocked into the nearest mickey D's to eat it
3) McDonalds has a very good history of fusion dishes & though the others do just as well, McDonalds is number one here, so it doesn't hurt them to switch the usual cheeseburger with Coca Cola, regular fries, ice cream cone and apple pie with something unique, shouting "Hey ! Limited Quantity !!!"

This applies to any other unique local variations being popular in your country folks, as long it is a multinational fast food joint. Next please