Why did North Korea become communist?
Long story short: The Korean peninsula was split into two following the Korean war (1950-1953), and the north was put under communist control.
See my longer answer below... (Pretty long)
The Korean peninsula became split into two "factions" following WWII- not really officially, but there were two people who claimed they were the leader of the country. These were Kim Il-sung(North) and Syngman Rhee (south).
Kim Il-sung was communist, following the principles of Marxism-Leninism and was inspired by the Revolution that took place in China in 1949, and wanted something similar to occur in Korea.
Syngman Rhee was anti-communist, but he was not very democratic either in comparison to Il-Sung, but he was more of a nationalist/conservative and was strongly against Il-Sung.
Growing tensions would escalate and eventually, it ended with "North Korea" invading "South Korea" in 1950, starting the Korean war- and this prompted the USA to enter the conflict by appealing to the UN (part of their policy of containment of communism following WWII)- Which then sent troops to aid South Korea.
While the UN coalition saw success at first and they were very close to pushing all the way to the Chinese border by the Yalu River- but China intervened, partly due to fear of US influence close to the border.
Due to Chinese intervention the UN forces were pushed back and the war came to a stalemate- but eventually, a ceasefire was negotiated (but not a peace- North Korea and South Korea are still in war with each other, at least on paper)- This split Korea along the 38-th parallel and set up a DMZ (demilitarized zone)- and Created North and South Korea, and that is basically why North Korea is communist.
Because the war could not conclude with a clear winner, the country was partitioned into two pieces- the North, later supported by the USSR and China- and became communist (well, "communist" today is debatable- they follow the principle of "Juche"- so to what extent they follow Communist principles is debatable) and the South, later supported by the USA.
The North became communist because this was the part of Korea which had the strongest influence by Kim Il-Sung, and also bordered with China, which acted as an ally and as another local communist nation.
Hopefully, this helped!
Russian and Chinese influence.
At the end of world war II the victorious nations divided up the territories of the defeated nations to be administered until the conquered nations were able to rebuild their governments and economies. Germany was divided into four part administered by France, England, America and the Soviet Union. Korea was divided into two parts the north under the administration of the Soviet Union and the south under the administration of the United States.
The Soviet Union supported a communistic government in the north that the Soviet Union controlled. American supported a democratic form of government in the south that America controlled. The United Nations mandated that an election be held to determine the future of the Korean peninsula.
The elections occurred in the south but were not allowed in the north. The Soviet Union provided military training and materials to the north. America provided economic training and materials to the south. The North had a strong military. The south had a strong economy. The north remained under the control of their Russian sponsors. The south remained under the control of their American sponsors.
North Korea used its military might to invade South Korea and unite the two parts back into one nation. America and the United Nations intervened to protect South Korea and then invade North Korea to unite the two parts into one nation. China intervened to protect North Korea and preserve the Communistic rule of the North driving the United States and United Nations back to original borders established at the end of World War II.
North Korea became communistic because of the influence of Russia after the end of World War II. North Korea remained communistic because of the military intervention and Chinese influence during and after the Korean War. ( which actually has never officially ended both sides still claiming control over the entire peninsula.)