If we use the subjectivist analysis of the term 'good' then "X is good" describes the attitudes or feelings or something else about the speaker. If person A sincerely says "X is good" and person B sincerely says "X is not good" they are both correct. The situation is like that of two people on an airplane. One says "I am nauseous" and the other says "I and not nauseous." Both are speaking the truth. If this is the correct analysis of what it means to say "X is good" then there are no moral disagreements at all. But there are serious moral disagreements all the time.

So on the subjectivist analysis, we cannot account for why we think there are serious moral disagreements and why they matter to us.