That is more vague than "Why is my car not going fast?"
Maybe the owner has put restrictions on it, requiring a password to use it, there's a lot of reasons. A better way to ask your question would be to say:
I have tried to access 188.8.131.52 and my browser (tell us what browser) gives me an error about this site being restricted. What does this mean, or better yet, how can I contact the webmaster to ask for permission to view it?
Something like that. But just "Why would an IP be restricted" is not going to get you very good responses. Maybe it has a formula for producing Thomas Kemper Cream Soda, and only certain people can access it... The list goes on.