Why?
I haven't played the game so I can't really speak for Snow and Serah. What I've seen of the game and the promotional material makes me feel that Snow/Serah is presented as a great romance - and maybe it is, but to me, Balthier and Fran are the ones with the perfect relationship. He's not just her lover but her comrade. She is 110% his equal - and she doesn't need protecting. When I first started playing the game I was irked by her "bunny girl" appearance, and there's no denying that her arse is pure fanservice. But what I like about her is that she subverts her trope: she's a fierce animal, a warrior, and in the myst (mist?) she becomes a mad March hare. She's anything but soft and cuddly. Most men couldn't handle her - they'd want to clip her claws and cage her in some way. Balthier's respect for her as his partner and equal, despite the fact that she's a) female, and b) a different species, speaks volumes for his own manliness and strength of character.
Next to Balthier and Fran, Snow and Serah seem a ittle banal.