Well there were a number of factors at play. First, domestically Britain was hindered by pro-America politicians and figures. Secondly, there was the support given to them by the French and Spanish, to a lesser extent the Prussians and Dutch as well. Without foreign support they would have eventually floundered. It was Saratoga that really clinched it. After that it was a retreat to New York to stave off any French incursion and refocusing on the Southern colonies which were perceived as having more Loyalist sympathies. Also there is no Royal Army and there hasn't been one since the end of the Civil Wars.