How and why did Britain's imperial policy towards American colonies change after 1760?
The primary reason for change was the costs of the French and Indian Wars, and the acquisition of former French lands in North America which were now part of British America by the treaty of Paris of 1763.
The size of British America was substantially increased by the Treaty of Paris, and Britain was concerned with the new area's governance. It was decided that the land was best reserved for the Indians; therefore George III issued the Royal Proclamation of 1763 which prohibited settlement in the West:
We do therefore, with the Advice of our Privy Council, declare it to be our Royal Will and Pleasure, that no Governor or Commander in Chief in any of our Colonies of Quebec, East Florida. or West Florida, do presume, upon any Pretence whatever, to grant Warrants of Survey, or pass any Patents for Lands beyond the Bounds of their respective Governments. as described in their Commissions: as also that no Governor or Commander in Chief in any of our other Colonies or Plantations in America do presume for the present, and until our further Pleasure be known, to grant Warrants of Survey, or pass Patents for any Lands beyond the Heads or Sources of any of the Rivers which fall into the Atlantic Ocean from the West and North West, or upon any Lands whatever, which, not having been ceded to or purchased by Us as aforesaid, are reserved to the said Indians, or any of them.
Additionally, the wars had been very expensive for Britain, and George Grenville, the King's Treasury minister, believed that the colonies should pay some of the costs of the war. He at first tried to raise additional taxes in Britain by means of a Cider Tax, but this was so unpopular he was temporarily removed from office. The Stamp Act was Grenville's first attempt to raise money in the colonies primarily to pay the cost of the war. Additionally, he ordered increased enforcement of the Navigation Acts by Customs officials who had largely ignored smuggling by the colonists.
The end result was the Britain's previous unofficial policy of Salutary Neglect was ended suddenly and unceremoniously.