Why did the patriot movement wane in the early 1770's?
To the extent that the patriot movement did wane in the early 1770s, it was because things seemed to be getting out of hand. The violence of the early '70s, seen in such things as the "Boston Massacre" scared many people and caused them to pull back somewhat in terms of the radicalism of their beliefs. At the same time, the British Parliament did some things to ease the tensions.
During the time from the Boston Massacre to early 1773, things were relatively calm. Parliament repealed all of the Townshend duties except for the one on tea. Even then, legal tea was cheaper than the illegal tea had been and was actually cheaper than tea in England. These sorts of things led many colonists to feel they could get along with England and that they should tone down their protests after the violence of 1770.
Of course, this did not last long and 1773 saw another eruption of tensions, one which eventually led to war.
In the aftermath of the Boston Massacre in 1770, the British government adopted a conciliatory stance toward the colonies. They withdrew troops from Boston, which did much to defuse tensions in that city, which had been the epicenter for resistance to British imperial policy. They also, under major pressure from powerful domestic merchant interests as well as colonial merchants, repealed most of the much-resented Townshend duties, with the tax on tea a conspicuous exception. This relaxation in policy led to a bit of a detente between Britain and the colonies which was broken by the attempt to force Bostonians to admit a shipment of tea in 1773. This led to the famed Tea Party, which led to a harsh response from the British government in the form of the Boston Port Act and the rest of the so-called Intolerable Acts.