1. Money. Most museums have no money. They either run on donations, on subsidies, or at the whim of wealthy patrons. They are very costly to run, especially the big ones. They are often in prime real estate areas, many require tight climate control, many also require specialised lighting to protect art etc.
2. Curators often see "taking care" of the exhibits as more important than actually exhibiting them. Not to mention they're often art/history majors with very little clue about anything digital.
3. Because museums are often subsidised, many of them are required to go through public tender procedures to get anything done. Because this is a huge pain for everyone involved, the results are often shit, as it attracts a certain kind of company to do the work. One of the tenders my startup looked at involved not only supplying the hardware and software for an interactive exhibit, but also the lighting and reinforced glass casings for various items. This was not our cup of tea, and the tender would subtract points for using subcontractors...
Personally I'm not interested in museums that are just glass cases with stuff without any explanation. Maybe a little paper legend is sufficient, but I actually prefer a screen which offers more info in the form of adio or video in multiple languages.
Depending on the exhibit, 3D printed replicas can be great as well.