I read somewhere that Thailand is not considered an earthquake zone and therefore the buildings do not need to be quake proof like other countries. I can't find the article again for a link though,sorry.
The fact Bangkok was once a swampy delta makes me wonder if liquefaction (water rising towards the surface during a quake and making it unstable) had it's part to play in the building coming down.
A terrible tragedy all round, RIP to those that lost their lives and hopefully those still missing are found alive.