Westerners generally, and Americans specifically, don't realize how their constant harping on "basic freedoms" comes across as ethnocentric. My parents are American citizens, but they were raised in Bangladesh and they don't really believe in free speech or democracy. My dad always talks about free speech with implicit scare quotes, like he’s referring to an american custom.
They revere Bhumibol, not his philandering, mercurial, and ripped son Vajiralongkorn who is de facto in exile in Germany. Everything in Thailand is de facto run by the military junta and aligned oligarchs like the Chearavanont and Shinawatra families.
And the younger generation (Gen Z) doesn't have much affinity for Bhumibol either, because they grew up in the midst of a middle income trap - their lives are better than their neighbors in Myanmar, Laos, Cambodia, or Vietnam, but CoL and the employment market is hellish, oligarchy and relations matter so if you didn't attend the right schools you're screwed, and abuses of power like the RedBull Heir running over a cop and all the extravagance around the royal family and their extended retinue grew more unpopular.
Tbf, I assume your frame of reference was the 1990s, and until the Asian Financial Crisis in 1997 Thailand went through a massive economic boom so satisfaction with Bhumibol was high. Bhumibol also at least tried to appear like he cared about normal Thai people.
Imo, it's the other way around. Thailand wasn't able to build strong institutions as that would have meant devolving power from the Military, Monarchy, and Crony Capitalists. This meant that economic reforms that would have helped Thailand recover from 1997 were not enacted as they would have undermined a lot of well connected and powerful people.
South Korea was roughly comparable to Thailand in the 1990s (and one of my professors who worked on Korean democratization confirmed this back in the day), but the IMF and US forced Korea to enact harsh reforms that helped them recover by the 2000s and become a developed country.
Also, a number of Thai business families were ethnic Chinese with ancestry in Guangdong, so a number of those families like the Chearavanonts decided to invest in China (the first privately owned companies in China were all Chearavanont funded because they had familial relations with the post-Mao leadership in Guangdong) [0] instead of in domestic R&D, while Korean chaebols didn't have a similar option and preemptively began investing in R&D in the 1990s.
[0] - https://www.nytimes.com/1995/11/14/business/from-chickens-to...
The son however - I’ve rarely seen his picture hung in homes or shops - just his father.
The truth though is Thailand has been run by big last name power as a structural thing. While Thai people generally embrace liberal humanism due to their Buddhist beliefs, the elite social structure still tries to hold onto the slavery based society of the past. The police are the primary fulcrum of their power, in a cross relationship with organized crime. The military waxes and wanes in its control, but it’s the police and dark powers that truly control Thailand.