Because sg is a letter combination that doesn't lenite. S will lenite on its own when it's followed by a vowel, but not when it's part of a letter combination and followed by a consonant.
This is explained in the notes to the "Phrases" still, the second one on the tree. This is what it says.
"Singular feminine nouns usually cause this lenition (in writing) in adjectives starting with the consonants: B, C, D, F, G, M, P, S, and T But not in those beginning with: L, N, R, SG, SM, SP, ST, and vowels."