Why did Britain switch to a Southern military strategy? Why did that strategy ultimately fail?

During the Revolutionary War, the British put into full force a military plan called the Southern Strategy once France became an American ally and entered the war in 1778. At this point, the British had to revaluate their plan, and under the leadership of British Secretary of State for the American Department, Lord George Germain, they decided that they would focus their efforts in the Southern colonies of Virginia, Georgia, North Carolina, and South Carolina, where they believed they would have the support of Loyalists—colonists who supported British rule.
However, the British did not ultimately receive the degree of Loyalist support that they had counted on. The brutal slaughter of American forces by the British cavalry under Colonel Banastre Tarleton and the very cruel practices that took place in the South under his leadership incited great resentment, and the Southern Strategy backfired.
Patriot guerrilla attacks on British supply trains and the morale-boosting American victory of 1780, the Battle of King's Mountain in North Carolina, further ensured the failure of the British Southern Strategy.