I can immediately think of three possibilities there, with plenty of room for more: 1) Garak wasn't likely to be attracted to the daughter of Dukat. 2) Garak was bothered by the age difference. 3) Ziyal wasn't Garak's type (various ways that can be taken). Not that a specific reason for why Garak wouldn't be attracted to Ziyal is even needed. Sometimes people just don't click.