Museums love visitors as they bring in revenue. However, they compete against sporting venues, the cinema, zoos, amusement parks, theatre, musical performances for disposable capital. That disposable capital in the attending middle class is declining as the economy continues its contraction. Furthermore, most museums lose money as operating expenses (power & other utilities, staff) aren't met by the revenue generated. They rely on foundations, endowments, fund raising and donations to sustain themselves. Even with that, most museums cannot survive without a dedicated corp of volunteers who man the service desk, information desk, serve as docents/guides and in some places, gallery attendants.
Space restriction is what prevents almost every museum from displaying its entire collection. Anything displayed must have a lot of protection. Sunlight damages fabrics. They must be protected from insects and small animals (think wool and leather). Theft prevention and fire protection as well as temperature and humidity control in a public area (more expensive than a storage room).
Museums need your support. It was found that people who go to museums are the same folks who as children were brought to them as a family outing. Take your kids and expose them to museums.