i guess the 'metal' thing is not that popular in south east asia. not only strao, but neither did other metal bands come there frequently... (correct me if i'm wrong. i don't know that much about singapore, just assume it is more or less the same as HK)
but they did went to HK once. and Taiwan. many other bands start to put HK in their gig lists too. so singapore has its chance as well.

imo singapore's chance is much bigger than...let's say, Indonesia

at least you don't need to deal with the religion issues~
but yes, culture and traditions are huge boundaries sometimes. that's the case of most countries in south east asia.