@Mick96 we don't know. It's sort of like why did publishing company A decide not to pick up series B? 😕
From what I understand, Naver can form a contract with the artists and pay them to work on their series full time/professionally (someone correct me if I'm wrong)
Being rejected means he won't get paid to work on the series. He did mention in his blog that he considered continuing the series and posting it in naver's challenge league or posting it on the blog for his fans, but he was focused on getting a different series approved by Naver (which btw that series also got rejected in the end). So we don't know what he will do next.
Just pure speculation/opinion here: I figure he's trying to get a series approved since that will be the source of his income. In the author's afterword, it's implied that finishing the series meant he was now unemployed, so getting paid for his work and getting food on the table would be a higher priority than doing it as a hobby for free.