I think it's just that Minthara is hotter than Shadowheart. She is more conventionally feminine in appearance, at least.
Damn, just realised that the male companions -- Gale, Astarion and Wyll are all attractive in their own ways.
Meanwhile, the only human(ish) female companion is Shadowheart.
Aside from her, we've also got Lae'zel, a githyanki, and potentially also a tiefling and a halfling. A lot of fetish material if you're into that kind of thing. lol.
Minthara is starting to look pretty good. Sorry everyone. I'm gonna have to turn her good like I did with Morrigan from Dragon Age.
It's just science.