The author says that national parks exist so that people can experience America; if they were to remain hidden, their value would be lost.
In order for America to be experienced, these parks must not remain "hidden gems." He argues that these parks "contribute uniquely" to the landscape and is a reminder of "what was" -- i.e., an America that no longer exists in modern
time.
He says that he wants others to experience what he has worked so hard to save. Therefore, these does not want these parks to remain "hidden gems."