Electronegativity is a chemical property that describes the ability of an atom to attract electrons (or electron density) towards itself in a covalent bond.
Sulfur and chlorine are in the same period (row) of the periodic table.
The nucleus of chlorine is larger than that of sulfur.
Because of the larger size, the nucleus exerts a stronger pull on electrons. This stronger pull translates into a higher electronegatively for atoms in the same row of the periodic table.