Someone had to die to demonstrate the Joining's risk -- otherwise, if everyone just survived, you'd almost forget it and it's supposed to be a key Warden thing. It can't be Oghren, since he's a returning companion with an epilogue and everything, so including an option to recruit him only for him to just die would be a massive kick in the face. It can't be any characters who aren't immediately likable (hey Velanna), and for similar reasons it has to be someone with a relatively deep character and potential (but preferably with a non-ridiculously in-depth backstory) because otherwise you don't give two piles of feces when they die.
Then, of course, it comes down to who they wrote first. It could be they made all the other companions first, finalised those, then decided to make a newbie to die. In which case, someone has to be written to just die -- and in that case, it just by luck fell to her. Because they're not going to introduce a new companion because companions mean more writing, more voice acting, and all that jazz, costing both money and time they might not have.