This teaches us that there is a seder, an order, in this world. Not everyone has the same strengths, the same potential, or is on the same level as anyone else. Everyone is important for a different reason, thus every individual has their own mission. However, we are not all here to operate on our own; there is an order. Everyone must take from someone else. This was Korach's mistake. He told Moshe and Aharon רב לכם כי כל העדה כולם קדושים ובתוכם ה' (טז ג) He felt that everyone is holy, everyone is equal. That's where his mistake was. He didn't understand why, if everyone is the same, would he promote someone over the other.
However, the sun and the moon demonstrate the exact opposite. They prove that this is far from the reality; there are differing levels of greatness, and those who are lower must reach to those who are higher. When lacking this knowledge, as by the moon, if he is not ready to accept from the sun, he will not be able to shine further, ultimately not being able to perform his tafkid.
Being that Korach's argument was against this idea, taught to us by the sun and moon, they were not willing to shine then. Korach's lack of willingness to be makabel made him unable to complete his mission in this world, hence they were unwilling to complete theirs now. Hashem counterfeited the argument, saying that it already started earlier with them. He reasoned that "If people are worshipping you, it shows that they are not prepared to accept from Me."