Because of the nature of the site. It is for people with questions about people they are interested in, that don't have other outlets for getting the answers. And the people that have the most questions are those that have the least experience.
So there is naturally a big draw for people that have not had a lot of sex, or for people that have been having conflicting feelings, perhaps struggling with natural desires vs religious upbringing, or with issues regarding sexuality or fetishism.
I don't what the ratio of virgins to non virgins is on here and frankly I don't care. Furthermore, there is also the issue of weather or not someone is being truthful about their sexual past. I suspect there are just as many ( if not more ) lies about claims of things that people have done as opposed to what they have not. To attempt to answer the question, my only guess would be that looking at the ages of many of the posters, there seem to be a lot of users who range between 15-21 years of age.. Not uncommon for people to still be untouched at that stage in life.