Vgn's description is correct from the balance/mechanic stand point.
If you're talking about character reasons...
Half giants tend to hurt people as children. They are incredibly strong. Imagine a child with the strength of an adult or a teenager that could pick up the rear end of a car. They are that kind of strong.
That tends to make them very withdrawn after hurting a few people out of accidents.
They also have to deal with the heritage of bloodlust in many cases which means many halfgiants have an really bad temper they have to keep in check.
Things like this, while not really having anythign to do with the physical looks, is why the half giants charisma is lower. They often lack self confidence, they are often shy, introspective and prone to wanting to be alone for fear of breaking people.
Does that help?
