Because most people are unaware just how sophisticated and prosperous Korea was before the 1800s (as well as the rest of Asia for that matter ... please read "The Great Divergence" by Kenneth Pomeranz; though not an easy read, provides a lot of data and statistics to back up his assertion that Asia - particularly China, but mentions Korea, Japan, and SE Asia as well - was on par with Europe on a per capita basis prior to the 1800s)
Though China invented movable type, Korea was able to use it a to a wider degree, greatly aided by the eventual adoption of hangul. Korea perfected the use of metal type, as noted by historian Henri Jean Martin. Martin, who specializes on the history of the book and printing, notes that Korea's system of metal type was as sophisticated as Gutenberg's but developed over 50 years earlier.
Militaristically, Korea was on par with China and Japan from the 15th-17th centuries (as well as Europe ... most likely surpassing Europe in certain areas). Turtle ships, anyone ... iron clad ships roughly 300 years before their development in the West. Not to mention cannons, guns, and rocket artillery. That is why Korea, despite having a lower resource base than China (though arguably not Japan), was able to maintain independence for so long.
It was only until Japan industrialized was it able to surpass Korea technologically and colonize it. Of course, to be fair, Japan was probably more culturally and economically advanced than Korea.
Japan was definitely not a backwater, as some people on this board like to paint it before the Meiji Restoration. During the Tokugawa period, Japan's wealth per capita was one of the highest in the world. Reference is a PBS documentary on the Tokugawa period as well as references by Pomeranz ... I know Angus Maddison is widely referenced, but I question his method ... I believe he does not take into consideration PPP (which is a much better method when doing historical comparisons) and instead does a straight nominal comparison, which honestly, how can you do that with historical data? Pomeranz uses a PPP method as well as comparison in consumption. Japan definitely was economically developed, in a pre-industrial sense (prior to 1800, the most prosperous nations/regions in the world were China, Japan, England, France, Netherlands, US, Korea, and SE Asia).
Furthermore, culturally, Japan was very developed, as evidenced by kabuki, noh, and the tea ceremony.
However, in general Korea was as capable as its two "neighboring giants". In fact, if you were to look at Asia from a 1700s/1800s perspective, Korea would widely be seen as an equal to its neighbors, depsite its relative small size ... think Netherlands surrounded by France and England.