If a congregation is having all the book studies meet at the hall, it is most often because
1. No one's home is big enough to host a book study of any size
2. Lack of parking in the neighborhood may be an issue
3. No one qualified spiritually to have one in their home
4. Most often, no one wants the hassle of having the book study in their home
The house must be cleaned for company twice a week and many sisters work and have a family to care for.
Some brothers and sisters cannot be trusted not to snoop or "borrow" items from the home.
As has been mentioned, the host family can never miss the meeting. Who wants 15 to 20 people trapsing through your house while you are tossing your cookies in the bathroom.
BTW, book studies in the home are required to have a 3-sectioned contribution box per a BOE letter a few years' ago. Some elder bodies may not have implemented it.