It's a famous restaurant. You can google the history, but basically tables were "given" to friends of the owner, and you can only eat there if you're invited by someone who has a table. You can't make a reservation. That's how the place has earned its cachet.
There's no lack of great Italian restaurants in NYC. And plenty of places that are happy to have you dine in their restaurants. I find it strange that people are attracted to the place simply because it's impossible to get in.