WARNING SPOILERS
Each vampire that has a "special power" is said to have aquired it due to having a heightened human sense, for example in edwards human life he was very good at reading people and so when he is a vampire he could actually read peoples minds. carlisle in his human life was very compassionate and so as a vampire has amazing self control needed in order to help humans. jasper was very good as a human at getting people to do what he wants so as a vampire he can literally control peoples emotions
now...bella as a human had a private mind. like edward described her mind in twilight: its like her mind runs on a different frequency to everyone else. and bella makes a subtle reference to the fact she thinks her mind is different way at the start of twilight.
*SPOILER*
so when bella got turned into a vampire she took this unusual human ability with her and it turned into a a special power- she is a sheild. she shields her mind from any interference and later on she learns she can extend this to shield others. so the reason is, is it sets up for the 4th book where bella has the special power which leads the cullens to victory!
Bella has a very private mind. She can't be touched there. What Edward and Aro do is clearly a mental thing; Jane, also, works inside the head (Jane doesn't actually inflict pain on anyone's body, she just puts the illusion of pain inside her victim's head. It's a very effective form of torture). Conversely, what Jasper does is no illusion. He affects the physical body, slowing the pulse and upping the endorphin levels to calm someone, for example, or raising the pulse and pumping out the adrenalin to excite them. Alice, also, works outside the mind in the realm of possible realities. She doesn't see the thought process behind the decisions, just the outcomes.
