Because it's part of the gameplay. Music contributes to the overall feel of the level. You can't just say what percentage of the whole feel it is, but just try to play a (preferably singleplayer) level with music turned off. Not as fun, is it?
Are you sure this is an obvious universal statement? Sometimes it really takes a lot of time to pick the right song, especially if you have a large collection to look through (which is always good to have, especially considering the size of an average tracker module).

Also, keep in mind that a review is not some sort of oracle, it's more or less an opinion. You might take suggestion from it, but do not take reviews (especially non-professional ones, the only kind there is on J2O) as the truth that you must apply. If there are points given or taken for things you don't bother with, just take note of this when looking at the overall score and make the final decision yourself. And if it's your level that is reviewed, a fraction of a point down because of a bad music choice is not the end of the world, and maybe next time you'll choose better.