Why does Thomas Jefferson focus more on King George than Parliament and the British people in the Declaration of Independence?

Because King George was the authority figure. Nearly every wrongdoing subjected to the colonists were because of him. He could have allowed colonial representation in the Parliament, be he didn't. He could have chosen not to pass the Quartering Act, Stamp Act, Tea Act, Coercive Act, etc. He could have chosen not to deprive colonists of the benefit of a trial by jury. The list simply goes on and on.