Very careful diplomacy, and relative lack of proximity.

All page references in this post are to Robert Bruce's “KING MONGKUT OF SIAM AND HIS TREATY WITH BRITAIN.” Journal of the Hong Kong Branch of the Royal Asiatic Society, vol. 9, 1969, pp. 82–100

Burma was right next to British Bengal and unfortunately, was ripe for territorial disputes since the lucrative entrepots along the Burmese coast were exactly the sort of places the British were interested in.

Thailand (Siam at the time) learned from the Burmese experience. It's notable that directly after the first Anglo-Burmese war, the initial coolness of treatment that had been given to the initial 1822 British Crawfurd mission to Bangkok was replaced by a much more conciliatory mood with the 1826 Burney mission (p.85). Bangkok thus consistently gave concessions, both territorial and commercial to the British and later to other Western powers. In the following decades the Thai crown began to infringe upon the free trade agreements and in 1850 Bangkok rebuffed a British mission under Brooke (pp 87-88) but Brooke's recommendation for war was tempered by the fact that King Rama III was dying and his likely successor, Prince Mongkut was known to be much more amenable. Mongkut ascended the throne as Rama IV in 1851 and made active effort to court Western favour (pp 93-94). In 1855 he hosted the Bowring mission which cemented Anglo-Siamese relations and formed the template for a number of other treaties with Westenr powers that followed.These basically secured Siam's status as a friendly neutral state, offering European powers extraterritorial rights and paving the way for Rama IV's successor, Chulalongkorn, Rama V, to go even further by actively inviting Western experts in as consultants. These were recruited from a variety of countries so as to avoid colonial reliance on one single power" (pp 97-98).

So basically unlike Burma, which encountered the British early on and had direct points of conflict, Siam was isolated enough to have time to learn from Burma's mistakes and had monarchs in Mongkut and Chulalongkorn who were canny enough to actively court foreing diplomatic relations.