Probably a combination of factors:

* 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.