Most of our national parks are located in remote places that have very limited telephone service and the telephone company provides internet service. Add to this the multiple recent years congressional desires to cut spending and you discover that the park service has no money to provide internet service to visitors. It's actually worse than this at some parks. One I know of implemented congressional mandated changes to the staff's email procedures then discovered that their internet service and local server systems would not support the staff's accessing email. Of course, congress will not appropriate additional money to upgrade the internet service or replace obselete hardware with equipment that meets mandated needs.
Some parks have internet service provided by the consessionaire, the people running the souvenir shop. If they are doing this, it will most likely be only at the headquarters building, and may be available for limited hours each day. If there's a consessionaire run RV park or campground in the park, they may also have wifi service available for a small (or large) fee.