Japan and Thailand successfully resisted Western colonization during the 19th century through policies of modernization. In different ways, both countries were forced to open up by Western powers, were subjected to unequal treaties with them, and implemented far-reaching policies on administration, economy, bureaucracy and education.
But by the end of the 19th century Japan became a powerful industrial empire while the same was not happening with Thailand. By WWII, Thailand became a client state of the Empire of Japan. So what happened?
Thailand avoided colonization because it worked as a buffer between British India (including Burma) and French Indochina, and made serious efforts to legitimize itself to its potential colonizers. It developed a standing army and engaged in many formal legitimization practices (like mapping its territory with western standards).
During that time period Japan had roughly 7x the population of Thailand, if we are talking power then its quite clear that the two nations are not the same in terms of potential to become major world powers.
On top of that a lot of prerequisite conditions of that industrialization and westernization (very much a non-settled issue) were seen with Japan, and there was quite a bit of luck. Japan was united, centralized, an island chain, and high in population while lacking the natural resources to make them a target (or at least were less lucrative than China). Japan saw what happened with western intervention with the Opium Wars and had the advantage of knowing how dangerous western powers intervention could be and the difference in strength (if it could happen to China it could happen to Japan). The call for reform (especially military) was much more united because of this. The Meiji Revolution is often romanticized as a battle between old and new but really both sides were more than aware of the need to modernize and had plans to do so, regardless the new government was particularly well willing to adopt European institutions and norms and had support. Japan being the first non-western nation to industrialize might also have meant less resistance for support as it wasn't seen as a potential rival.
The actual economic development of Japan is quite a bit more standard than most would expect. Japan is somewhat of a unique case because of its early development as a non-western nation but that's likely the best way to look at it. It had a similar population to a European power and developed roughly around the same time period. Ultimately they were only a decade or so behind a place like Germany (and 20-30 years behind France) for industrialization and Japan is often included as an early case of industrialization. Japan as a mirror of European states was also furthered by its effort to copy and adapt industrialization practices. Schools, government, banking, military, and plenty of other areas were targeted for westernization and people were sent to study and experts contracted to help. There was a willingness to completely change institutions and norms as well as central planning to go along with it. Government invested in coal and other areas, encouraging the organization of large business and financial institutions. Japan also immediately started foreign intervention, not even a decade after the Meiji Restoration Japan forced Korea to sign an unequal treaty and give Japanese industry an opportunity for cheap raw materials and food while opening up a market for Japanese produced goods.
Being powerful and being industrial are two separate (if still related) things so the lack of economic development for Thailand (compared to Japan) is a separate issue than its difference in power. A relatively low population nation that's also trying to play catchup will have a hard time becoming a major power, especially with all of its neighbors already colonized. I'm not as familiar with Thailand's economic history and development but a lot of the industrialization checklist is not there, its hard to measure the specific importance of individual factors but Thailand lacked some of the major ones especially the advantage of scale.
edit: Its important to note the window of opportunity for serious western intervention in east Asia was short while its intervention and eventual colonization in south east asia was much longer. After the Opium wars the reforms of China made it quite effective at fighting off any major intervention, it wasn't until the reform faction lost power and the military got defunded (with horrible corruption throughout) that China again became weakened enough for a 2nd phase of intervention.