What I meant was "surprise" that Thailand hasn't attempted to get a site in Bangkok.
I'm really sorry to misunderstand your point. I forgot Scotland when I did some research on this matter, yes it is another good exception.
For Bangkok I did some reseach last few months, it is actually some discussion to nominate some site in the city. From this link http://worawutboonwong.wordpress.com/2013/10/04/world-heritage/
There are 2 sites in Bangkok, Cultural landscape along the Chao Praya River and Architectural works of Prince Naris.
And from this news http://www.nationmultimedia.com/national/Heritage-status-eyed-for-Chao-Phraya-3020682 8.html
It is interesting that the nomination will be historical sites along the river including Wat Pho that is next to the royal palace, but nothing mentioned about the palace which is maybe the most outstanding site!