Why was King Sejong named "Sejong" instead of "Sejo"?

The Joseon Dynasty lasted 519 years, during which there were twenty-seven Korean kings, including Emperors Kojong and Sunjong. After their deaths, they were given "temple names" (廟號 - 묘호), which consisted of two characters. The first character could be called an "adjective character" because it helped to describe the king. The temple name for the dynasty's seventeenth king, for example, was "Hyojong (孝宗). "Hyo" (孝) means "filial piety," which suggests that King Hyojong was a dutiful son. The second character of the temple name was one of two characters, 宗 (종) or 祖 (조). Among those twenty-seven kings of the Joseon Dynasty, eighteen had the character 宗 (종) attached to their adjective character and seven the character 祖 (조). Two of the kings, however, were given the title of 君 (군), which was a title given to princes. not kings. Whether a king's name ended in 종, 조, or 군 was decided only after his death and was based on how he was judged to have performed his duties as king.

The characters 宗 (종) and 祖 (조) were titles of honor for Korean kings, but the character 君 was used for kings who had been dethroned and denoted back down to prince for dishonoring the throne. Though 宗 (종) and 祖 (조) were titles of honor, they were not of equal honor. The character 宗 (종) was attached to the names of kings judged to have been "virtuous," and the character 祖 (조) was attached to the names of kings judged to have been "meritorious." A "virtuous king" simply fulfilled his expected duties as king, but a "meritorious king" did something beyond his regular duties that earned him special merit, suggesting that kings with names ending in 조 were a step above those with names ending in 종.

The following is a list of the Korean pronunciations of the first characters used for the twenty-seven names of the Joseon Dynasty kings. It might help you memorize the order.

태정태세 문단제 

예성연중 인명선 

광인효현 수경영 

정순헌철 고순

After memorizing the list, then you just need to learn on which names to attach 종 (宗), 조 (祖), or 군 (君).

The first king of the Joseon Dynasty was King Taejo (太祖). The attached 祖 means he was considered a "meritorious king." Apparently establishing a new dynasty was quite a feat, earning him his special merit. The character 太 (태) means "great," and was the same character used for the founder of the Goryeo Dynasty, whose name was also "Taejo" (太祖).

The next Joseon Dynasty king to have 祖 attached to his name was King Sejo (1455 - 1468), the seventh king of the Joseon Dynasty and second son of King Sejong the Great (1418 - 1450). The first son of King Sejong was King Munjong (1450 - 1452), who died shortly after becoming king, passing the throne to his 12-year-old son, King Danjong (1452 - 1455). Though King Sejo had forced his young nephew to abdicate the throne to him and killed many people, including his nephew and younger brother; King Sejo apparently earned enough merit by strengthening the monarchy and improving government administration to have 祖 posthumously attached to his name.

Even though King Sejo demoted King Danjong to prince, thereby, reducing his title to 君 (노산군), he is now referred to as King Danjong because scholars during the reign of King Sukjong (1674 - 1720) felt that he had been undeservingly dethroned and, therefore, they restored his title and gave him the posthumous name "King Danjong."

Anyway, the point I want to make is that even though King Sejong the Great is now credited with many achievements, apparently none were considered great enough at the time to merit 祖 being posthumously attached to his name. I find that somewhat interesting.