It may be due to what the guy asked for

Some guys are only comfortable with a CBJ, while others prefer BBBJ.  You would have to discern from the reviews if that is indeed the case.  In other circumstances a lady may have started out with only CBJ and at some point expanded her menu to BBBJ.  And in some cases it may have to do with the guy, as not all show up "fresh" and that a CBJ may be the only option (and even then she may have passed on that as well??).  

As far as the ladies getting more comfortable as you see her more than once...that would be a fair statement to accept.  However, if a gal does not do BBBJ for her safety, you can not be sure that even with multiple sessions that would ever be an option.  If you are planning on multiple sessions you might want to chat about what may also be available to you.  Many of the ladies do have menu items that are not well known through the reviews.

Words from my escort

There is a girl that I have used frequently and asked her the very same question.  She is down to do a BBBJ for the rate she charges an hour which is $300/ hr.  I told her about my previous experiences with girls that charge $150 hour.  She said with her rate, her Johns are usually professional men that can pay for a higher rate and they are usually clean.  If someone is charging less than that then these girls through so many guys in a day, so the CBJ is the best option.  From now on, I will look for girls that are bangin hot and charge around the $300 range.