Is it just the easiest free pics to use? The mummies no longer available to study and tucked away somewhere? Or is it just that those pics were made when the mummies were first unwrapped and thus are the best pictures available? I just noticed that most are black and white pics from around 1880s-1920s.
Based off what I am aware of from working in a museum, all images taken at that time would fall under fair use, whereas newer images (specifically post-1970s) are automatically copyrighted to the photographer (or in this case the institution that owns the mummies, copyright of photos taken of collection objects is a fairly standard rule most museums include in contacts with photographers).
Most likely, these images are being used because they are fair use and because the black and white images are striking and convey the age and history of the object to a non-specialist viewer very quickly.
If you're interested in learning more about art copyright laws I'd recommend checking out Museum Administration 2.0, it has a very in-depth chapter about it.
Also, if you are mainly thinking of unwrapped mummies, there are far fewer modern photos of that because modern codes of ethics prevent us from unwrapping them (for good reason). For one thing, mummies are religious and funerary objects which means they need to be handled with extra sensitivity. For another, because they are so old and delicate unwrapping them is extremely detrimental to the long term stability of the mummy, which is exactly the opposite of what a museum professional or archeologist/egyptologist hopes to achieve.
In response to this dilemma, a method of scanning mummies using CT scans to allow further study without unwrapping them has been developed. I am linking a few articles about this below in case you are interested in taking a look.
https://pubmed.ncbi.nlm.nih.gov/26979539/
https://www.smithsonianmag.com/smart-news/pair-mini-mummies-contain-non-human-remains-180975443/