Because France really really really hated Britain. France really really wanted to reverse the fortunes of the 7 Years War. If anything, the British considered the American Rebels less of a threat than one posed by France. Which threatened Britain's survival more? The American Colony of 2 million of which only half the population wanted independence or the Kingdom of France with 16 million people and less than 100km away from Britain. Much of Britain's Navy, the strongest force of the British military was held up in Europe, trying to intercept and defend Britain against a Franco-Spanish Invasion. Although the invasion never materialised, the threat was very real. 40,000 French troops were raised at Le Harve.
http://en.wikipedia.org/wiki/Armada_of_1779
Had the full force of the British Navy been sent to the Atlantic, they could have focused more of the strength there and win. Most of the major Cities were on the coast and could had been captured by the British. A couple of months of stockpiling of troops and supplies and it was a matter of time until the British struck into the heart of America and took the countryside as well.
Are we talking about the same Britain who fought in India and succeeded in conquering a huge chunk of it against both local and foreign forces just 20 years before the revolution?
So what? When the Americans won Saratoga, the Brits won the Philadelphia Campaign. Both have far reaching effects. Philadelphia opened up the South for the Brits. Saratoga brought in the French. I remember reading that Clinton had to evacuate Philly because he wanted to strengthen the defense of New York against a possible French-American Attack. Both sides had excellent generals and effective use of them.
I would also like to add that after the Spanish captured Florida, it secured the southern flank for the Americans to head north. I know that its not a deciding factor in the Southern Campaign, but it did impact the course of war.
I'm not pointing at that, I'm pointing at how resources were diverted away from the US Campaign because the Brits were trying to defend themselves in Europe.