* Fewer adults interested in science than children. Children are learning new things. Magnets! Pulleys! Not many adults (outside HN) are going to get excited about a pulley.
* The people making the museums don't have sufficient scientific knowledge to do science for grown ups.
* Exhibits for children are much easier to make robust, and probably cheaper to make.
That said I do think it would be really cool if there was a science museum for adults. There's all sorts of things you could show.
* Fewer adults interested in science than children. Children are learning new things. Magnets! Pulleys! Not many adults (outside HN) are going to get excited about a pulley.
* The people making the museums don't have sufficient scientific knowledge to do science for grown ups.
* Exhibits for children are much easier to make robust, and probably cheaper to make.
That said I do think it would be really cool if there was a science museum for adults. There's all sorts of things you could show.