For Singapore, I think it is due that the place is a hub with a lot of international events and business tourism, the same kind as in Dubai or Hong-Kong.
For Kuala Lumpur I would say it is partly due to the fact that the airport is the home-base of Air Asia which is the cheapest airlines in the area and they have now flight from and to region outs Asia such as Europa or Australia. Plus, Malaysia has as many tourists as Thailand (around 20 millions a year). Also, Malaysia is one the richest and most developed country in South East Asia with a lot of immigrant from the surroundings country. If you combine those three facts that pretty much explains why a lot of flight to KL are quite cheap and why it is a hub.
For Kuala Lumpur I would say it is partly due to the fact that the airport is the home-base of Air Asia which is the cheapest airlines in the area and they have now flight from and to region outs Asia such as Europa or Australia. Plus, Malaysia has as many tourists as Thailand (around 20 millions a year). Also, Malaysia is one the richest and most developed country in South East Asia with a lot of immigrant from the surroundings country. If you combine those three facts that pretty much explains why a lot of flight to KL are quite cheap and why it is a hub.