1. The Star Wars saga is, essentially, the Skywalker family saga. It'd make more sense for the main character of the sequel trilogy to be related to that family, rather than an unrelated Palpatine (though I did like the idea of her essentially adopting the Skywalker name at the end).

2. The Star Wars saga is also a story of redemption through familial love. It'd make more sense for Kylo Ren/Ben Solo to be redeemed by his sister than a random love interest.

3. Her outfit and hairstyle in The Force Awakens are reminiscent of Leia. Though such things are obviously not genetic, they are often used...