Even though it's said by other heroes he should rank up, he refuses as he feels it is his duty to remain in the number one spot of A-Class to prevent the unworthy from becoming S-Class heroes, except for those whom he sees worthy like Genos, fitting Sweet Mask's philosophy about "heroes needs to be beautiful too"
But how does he plan to stop? Had he not approved of Genos, what power does he have to stop him from climbing to class S. Isn't that the decicsion for the association?
As mentioned in this answer,
When heroes are ranked #1 in their class they can choose to stay at the top of their class or start at the bottom of the next class.
There is no other way than being #1 to rank up. Therefore, as long as you are #1, no one in your class except yourself can rank up to the next class.
Sweet Mask is "supposed" to be way too powerful for the A-Class (from what he says, as we never saw him fight), so there is no way anyone in the A-Class would ever be stronger than him. Therefore, no one can take over the first place of A-Class as long as Sweet Mask doesn't rank up.
The only limitation of his system is that he can not decide on who can directly start as a S-Class, as it is only decided on the tests. This is why Sweet Mask could not influence Genos' rank
