Because THAT is HER "POWER/GIFT" that she possesses... like Alice can see the Future... that is "Alice's Power/Gift." (Or maybe it's that she's so stunningly beautiful that her looks alone makes all 吸血鬼 lose their senses, hence, they are unable to read her mind. Just a thought.... ;D )
because she has a shield from any mind influence of any vampires. Haven't 你 read Breaking Dawn? She learned how to use it and how to let Edward read her mind.