Because when you start to play good tennis players (of which Brianti isn't one) she will get destroyed. Hence her record vs top 10 players.
She's still young and inexperienced. Her record is poor (3-14), but that's to be expected. Give her time to grow.
I've seen Caroline against Ana and Jelena during the slams, and she can put power on the ball when she needs to. But against these lowly girls in the first round of an event, why should she be more than a pusher? She'll do more than push this weekend.