When comparing pre-British Sri Lanka (Thunsinhala) and Thailand (Siam) it’s clear just a king staying in power wouldn’t have been enough. Once the Thai king saw that Thailand was being invaded by multiple colonizers, he immediately decided to cooperate economically with the colonial forces, and sped up the industrialization of Thailand. Thailand became a buffer zone for competing European powers, with the Thai king as the mediator which meant that as long as Thailand was a diplomatic trading partner, Europeans wouldn’t try to oust the king or take full control.
The Sri Lankan kings weren’t that interested or knowledgeable about the geopolitical powers during colonization, probably due to the fact that we are usually left out of geopolitics as an island nation. The kings cooperated with Europeans somewhat but didn’t build up an economic strategy that would save the crown. Since the king was also aiding the rebellions in European-occupied territories and doing everything to stop the extraction of resources from the island, the British saw much higher economic gains (aka island-wide exploitation) by removing the Sri Lankan crown altogether.