I think your question really meant to be why did you join a BDSM forum. It happens that there is history to this particular forum, for most participants it was a second forum they joined, after Max Fisch.

Some answered the first question, why did you join a forum, others the second, why did you join Domina.

I will answer the first, because I think this is what you want to know. Most subs initially join the forum because they want to write about a mistress. Most of the time a positive review, sometimes to criticize.

After they join they stay because of the people. They can discuss subjects interesting to them and taboo to the general public. Some people connect on a forum too.