Why is the domain name "do.it" unassignable?

I was browsing both godaddy.com and namecheap.com looking for a stunning domain to buy for my next project, and one of my researches was "do.it", with the Italian TLD. Both website told me that it is unassignable. Why is that so? Is it a too appealing domain to be assigned to the pleb? It seems it hasn't any whois entry, it is just not for sale, and, for the sake of curiosity, I would be pleased to know why.