My family visited the AFA two summers ago. The chapel itself accomodates Protestant, RC, and Jewish services, as well as space for Islam, I think. The synagogue is breathtakingly beautiful, as is the whole structure. Maybe the Wiccans didn't want space there because their little spells wouldn't work. All that holy God mojo and all.....
Most Wiccans don't do "spells". More likely they preferred an outdoor space because Wicca is about nature-worship, and nature makes a better setting for that type of worship than a human-built structure.
No... it's because Wiccans don't believe in any need for a building, and in fact, believe that one cannot properly worship inside a building.