The difference between Korach and the others is that Korach came with demands, not questions. The others came to point out a problem they were having, that Moshe may not have thought of. They asked why they were left out, but they were prepared to accept an answer, including “This is what Hashem wants”, if that were in fact the case. All they wanted was for Moshe to please check with Hashem whether that really was what He wanted.
