Their rates are higher for the same reason that designer clothes cost more than generic: you are paying for the name and a higher quality "product".  Also, for the same reason that a medical specialist charges more for an office visit than your primary care physician: you are paying for their skill.

Seriously, I just spent a glorious hour with Christie Stevens last week (review posted).  Now, I have certainly seen non-porn stars who are just as attractive.  I have also slept with swingers who are just as attractive.  But, the quality of the session was so far beyond what the average escort, or the average swinger provides that I felt that Christie was worth every penny paid and more beside.

Now that's not always the case.  Some porn stars just aren't worth the money that they are charging and sometimes come with an attitude.  But, by and large, porn stars are *professionals" on a different level than the average escort in the same way that medical specialists are on a different level than general physicians.