KUL is in many ways more modern and larger than Singapore and Bangkok. It is the largest airport in SE Asia and also the home base for the largest airline in SE Asia, MAS. However, SIN has always been a traditional stopover for the Kangaroo route since the days of BOAC, even though BA operated to Perth from KUL. Even in the days of Subang, traffic loads were significantly smaller than BKK and SIN. Now superimpose that Subang traffic into the massive area KLIA currently is, makes the light traffic even more glaring and acute.
The Malaysian govt has foresight, because the traffic is expected to grow. Just that the airport was opened at the wrong time, right at the tail end of the Asian Economic crisis. On the plus side, Qatar Airways, Yemenia and a handful of carriers did inaugurate / increase services into KUL.
Now with pax traffic climbing back to pre-911 levels, MAS is also increasing frequencies to many of their destinations and they are currently the only airline flying the prime routes to LHR and CDG - fleet stream lining notwithstanding! However, I must attest to the fact that the peak periods at KLIA is a tad boring...
What KLIA needs is to win some awards like Changi has for the last decade or so, to put it on the international map.
Welcome to my starry one world alliance, a team in the sky!