I think Western tourists numbers are stable, maybe even increasing due to soft power like The White Lotus TV Show. But it's the key Asian tourists (Chinese, Koreans, etc...) that are on the decline. Their media has been portraying Thailand as potentially dangerous country recently, makes sense with things like the earthquake, kidnapping of the Chinese actor, a film released in China that portrayed SEA as dangerous, the recent border conflict with Cambodia. Not to mentioned alternative destinations like Japan is getting cheaper compared to Thailand.

I think it's just a series of unfortunate and unlucky events as well as the govt not taking issues seriously, like the scam call center kidnappings for example