I think she is just kinda the default if you have no imagination - softspoken human female that isn't totally obnoxious.

In terms of female romance options Lae'zel is the only one who it seemed to make sense would be forward enough to try and jump your bones after just 2 or 3 days. Karlach wasn't even an option for me, she had no romance adjacent dialogue at the tiefling party. I wonder if part of that was just how late she joined up in act 1, I did get her engine "fixed" for the first time but it was one of the last things I did and I can't remember if it was before or after clearing the goblin camp.

I did really enjoy Shadowheart's arc though, especially since I generally adopted a policy to not suggest or try to interfere with the companions' quests. Just left them to their own decisions without trying to sway them, and the fact that Shadowheart chooses to reject Shar and Lae'zel chooses to reject Vlaakith (or at least hear Voss out) made for two of the most powerful moments in act 2 IMO. There is lots to like about all three of them throughout their arcs and that's part of what makes them so great.