Because they want to tout the success of their self-isolation methods, which were strict long before COVID showed up. According to South China Morning Post:
Kee Park, a lecturer at Harvard Medical School who has made numerous
humanitarian trips to North Korea, said he believed Pyongyang wanted
to send the message that its self-imposed isolation had kept it safe.
“Their confidence in their public health measures also allows them to
take a more patient approach to see what, if any, issues arise with
these new vaccines as they get rolled out,” Park said.
Donations were similarly rejected during the 1990s famine in North Korea:
During the 1990s, the North downplayed food shortages until a
widespread famine that killed an estimated 240,000-3.5 million people
forced it to seek international assistance. How will ‘emaciated’ Kim
ride out a famine in North Korea? With China’s help 6 Jul 2021
“Saving face is a strong cultural motivator for Koreans,” said Dan
Chung, executive director of Crossing Borders, a US-based Christian
aid group that works with North Korean refugees. “Though it seems as
if North Koreans are reaching out for help, a moment later they could
indicate the exact opposite. This makes helping North Korea all the
As a final note, Kim Jung Un and his senior leadership are well known for their lavish lifestyle so we could speculate that they're all secretly vaccinated already, making them care a lot less about how many vaccines are available for the rest of their citizens.