Few reasons BBW, cinderellla and JP sell out fast could be, well, I could be wrong:
1. They are made available at certain stores/locations only. If next week, people find out MM and PT is not available at stores. May be the stocks will not last this long. eg. JP , it was released at very few stores and before online release, quite a number of people miss the chance and have to turn to online release.
I am one of those thought of checking them out at store before buying for the PT and MM collection, cream color base is the only one from MM collection, I am interested in.
2. The packaging and shades are appealing - I have to say packaging for cinderella and BBW are really appealing for me. JP definitely has the shades I really want to get my hands on after seeing all the swatches before release. While MM, I don't find anything that I feel like screaming "I want". The lip palette is unique, but I foresee it's not something I really will reach out that often.
I buy permanent one as well (normally it is by recommendation). With so many choices out there, I feel tired to go through and try each one of them.
3. Some could be fans of cinderella. or probably JP/BBW.