I'm assuming you mean available to the general public, run by the government? There probably are smaller, niche social networks for gov employees to collaborate. If there aren't there probably should be.
But to answer the assumed question, it's because there is no practical way they could compete with private companies. Sure, theoretically they could dump tons of money into developing cool new features, slick UIs, and obtain top talent. They would also need to spend billions to crush or acquire upcoming networks to maintain their dominance (Whatsapp, Instagram). Or legislate them out of existence. In practicality there is no way this would (or should) ever happen.
But to answer the assumed question, it's because there is no practical way they could compete with private companies. Sure, theoretically they could dump tons of money into developing cool new features, slick UIs, and obtain top talent. They would also need to spend billions to crush or acquire upcoming networks to maintain their dominance (Whatsapp, Instagram). Or legislate them out of existence. In practicality there is no way this would (or should) ever happen.