The reason that he had such long reign was that his father had died before George II so he inherited the throne from his grandfather and was therefore a comparatively young man when he succeeded. Britain had developed considerably during his reign from the agrarian revolution to the start of the industrial revolution.
Historians are now re-evaluating King George III and some thought is now being given to the possibility that he was more sympathetic to the wishes of the colonisst is America than is usually portrayed. Remember that Britain was still developing as a constitutional monarchy and that the king always acts on the advice of the ministers. The ministers were probably more conservative in their views than the king. The fact is that America required an army as a defence force but were not prepared to pay for it with out control over it, hence the slogan ‘no taxation without representation’. The new United States still did not like the idea of a standing army so they passed the Second Amendment to the constitution to provide for a militia, a fact which is still misunderstood in America to this day.
As for his illness, it is now suggested that he suffered from Porphyria which is actually a blood disease and this is what caused his incapacity