Why Ysolda?
I figured Lydia would be really popular...
With Ysolda, it's gotta be looks. If you take her character into account, she's so fixated on owning an inn that she's done some extremely shady stuff to land the money required to purchase it, including (but not limited to) drug-dealing. In addition to that, her dialogue indicates that she's the plaything of Mikael, the local womanizer. Humorously enough, he'll show up at your wedding, which makes things pretty awkward for the both of you.