Why didn't they modernize? They did! Rama III's reformed were continued, and by the 1900s, Thailand was a fairly centralized state, with a strong enough military to avoid being colonized (unlike all their neighbors) and Western-educated monarchs, with Western-style science and education slowly expanding to the rest of the population.
Why didn't they spectacularly take off? Out of the countries you mentioned, only Japan spectacularly took off all on their own, and Japan had five times the population of Thailand, a much more centralized and efficient government even before Westernization, and natural defenses that made them less of a target for colonization. South Korea (or just Korea in general for the time period) didn't modernize at all in the 1800s, was colonized by Japan, and remained one of the poorest and least-developed countries in the world until the 1950s. Singapore, too, didn't modernize at all in the 1800s, was colonized, and remained a third world country until the 1950s. Why have South Korea and Singapore (and Hong Kong and Taiwan) so rapidly modernized in the 20th century? ...that's outside the scope of this forum.
In short: why didn't Thailand go from marginalized to world power as quickly as Japan? For the same reasons no one else but Japan did. Japan was very unique, and compared to similar nations in Southeast Asia, Thailand's modernization went quite well.