Probably because many have only been to one resort and loved it so much that they dare not risk going elsewhere in case they didn't like it. They are both fantastic resorts and both have their own attractions.
Pamporovo is my favourite purely because I have so many friends there, and I wouldn't dream of going elsewhere.
There are not many places around in this day and age where you walk down the street and are recognised from your trip last year. This may have something to do with the fact that my 8 year old talks to whoever will listen - can't imagine where he gets that from