because the word begins with s + consonant; it's singular article would be "lo" and its plural article would be "gli"
See the first table in this link under "articles":
As you can see, it can get a bit complicated. The main thing to remember is "gli" is used with plural masculine nouns starting with vowels, the letter z and s + consonant. The others are less common and you can learn them as you go.
Gli is the plural of l' and lo. So you use it before plural words beginning with either a vowel or s + consonant or z