Why can't I view the User History for all my users?
You do not have the required scope assigned to you.
To view the 'User History' for all your users, you will need to assign the scopes that the users are using to yourself.
For example, if the users have Scope X assigned to themselves, you will need to make sure that Scope X is also assigned to you. Only once this scope is assigned to you, will you be able to view the User History.
To assign the scopes to you, go to the System tab > User Security page > and assign the respective Scopes to yourself.