What best explains why the British made the decision to end the war with the American colonies?

Continuing the war was becoming too expensive.

Explanation:

The British had already waged several wars for consecutive decades and had not been able to pay the war debts of the earlier wars. They already relied on the American colonies for the taxes so that they could repay the war debts. But being at war with the colonies itself cut their source of revenue entirely. Hence, it became difficult for the British to continue the war with the colonies owing to the heavy expenses.