Three reasons why? One reason could be that if you think about it, the colonists were provoking England, and King George iii stood up to take back control over their colony, trying their best to save them from themselves. The next one could be that Britain was that Britain needed to pay off war debts. It was the colonists' responsibility to help Britian, so when they weren't there when they needed them most, he had to stop their uprising but failed. I guess one more could be that other countries led Britian into fighting by helping it's colony, which is now modern day USA.
These are just suggestions, but I hope these help! :-)