Because, Trump.
Yes, because of one person, Donald J Trump. Many others have helped, Xi of China, Moon Jae-in of South Korea, Abe of Japan, and oh yes, Putin of Russia. Do not think for a minute that this is happening without extensive involvement by Russia. North Korea does more trade with China, but North Korea has more respect for Russia. North Korea has been a protectorate of Russia since WWII. But none of it would have happened, but for the strategy, style, knowledge, skill, will and determination of Trump. It's one of the most amazing international events of the past 70 years.