A number of reasons, I guess. Not that I would suggest that it was the right choice for anyone else, necessarily.
First, we're a club, I'm not always looking for or open to expanding membership. When someone is interested, I talk with them on the phone and try to determine whether we're a good fit for them or vice versa. I frequently at this point, suggest some other option for people, different style, different type of gym, maybe something closer to where they live, even other local Wing Chun instructors if I think they might be what they are looking for.
We don't have windows or a sign and I like that people don't necessarily know where we are. We're training, not marketing. I've got limited space and there really isn't any place for anyone to sit and watch if they were to want to do so. I don't teach kids, so there are never parents or rides waiting around for class to end. Our kwoon is in the basement of a commercial building and I'm responsible for whoever is hanging around down there.
I respect what someone said above about having some overbooking, but I don't personally do things that way. I have a small group and we come to train every time. I don't want anyone who isn't committed. Business travel, birthdays, etc not withstanding, we all count on each other to bring our best efforts in every class. I feel, as their sifu a responsibility to them and I don't want to take that on for someone who I don't know more about than I would learn from having them watch.
So, if I talk with someone on the phone who seems to really want to learn and seems like a good fit for us, then I invite them to join us for the night. I modify our plan for the class (another reason I don't like to do it too often), so that they get a good overview and sample of what it would be like to train with us. I tone down anything that might seem dangerous. I answer questions and we have tea at the end of the night, like we always do. I tell them before they come that at the end of the night we're going to thank each other and part ways and they can get back in touch with me if they are interested in joining the club. That way neither of us is on the spot in the moment. They're not obligated to me and I'm not obligated to them beyond this point.
I'll talk with them, I may go back and forth in email with them and we host them for an evening, that way everyone is sure what they're getting into before they join. I will also say that most people who contact me never make it in either because I don't extend an invitation or because they take the vibe and walk away. I'm responsible for running a good school and helping everyone learn and for doing what I can to ensure everyone's safety and this is my process.
Like I said, it may not be right for anyone else, but it works for me.