Hard to say why people leave the forum. One reason may be that they feel outsiders and see no contact, like one of the posters wrote that nobody has a word to say and does not address him/her directly, as if he/she were not there. Another reason is that they get bored and lose interest in the subject, or may never have had it at all and joined the forum just being curious and having a slight fondness of the band not enough to sustain them longer. Others might have more exciting things to do in real life or start getting busy at home or at work. For example, there was Mr Mustard but he disappeared, I liked his positive enthusiasm, though. There was Mr Dude, he disappeared after nobody had reacted to his post about censorship in Britain and it getting harsher. He was a very good poster. Mr G-mod also left for not clear reasons. What a pity! He was very-very nice. But these are all big boys who know what they are doing. It`s their choice.
However, there is another category of posters. They appear, write something about how happy they are to have found this forum. Then they immediately send their fabulous pictures, impressing everybody, and disappear, I guess, waiting for somebody interested to pm them, which was their aim in the first place. Yes, you are right, these have been ladies. I haven`t been here long but I`ve already noticed this tendency.
Also those who leave may find some forums more to their liking.