Long story short, America had a successful revolution and India didn't.
America had the American Revolution and won independence through escalation of protest.
The American Revolution was a colonial revolt that took place between
1765 and 1783. The American Patriots in the Thirteen Colonies won
independence from Great Britain, becoming the United States of
America. In alliance with France and others it defeated the British in
the American Revolutionary War.
India was granted dominion status because organisations arranged talks with the then Viceroy of India which then formulated into the Indian Independence Act 1947 to grant said status:
The legislation was formulated by the government of Prime Minister
Clement Attlee and the Governor General of India Lord Mountbatten,
after representatives of the Indian National Congress, the Muslim
League, and the Sikh community came to an agreement with the
Viceroy of India, Lord Mountbatten of Burma, on what has come to be
known as the 3 June Plan or Mountbatten Plan. This plan was the last
plan for independence.
Britain still needs trade, soldiers, and resources from its colonies and India was no exception, they played a huge part in the Textile Revolution and if they weren't going out without a fight, From Britain's perspective I'm sure they wouldn't need to be absolved all allegiance.
Indian colonization provided the key ingredients for the textile
revolution (also called the Industrial Revolution) in Britain. The two
key ingredients are cotton and markets. Cotton was massively imported
from India, and with an abundance of coal in the U.K., it was
processed to produce mass-manufactured textiles that were then
exported back to India (and other places). That is the core of the
Industrial Revolution that took the West to economic prosperity (source).
Note: as Sjuan76 points out in the comments, there wasn't dominion status back when the Americans were absolved, it started with the Canadian Confederation in 1867.
Thanks. Can you answer 2nd part - What are the reasons if possibly
inferred, by which Britain was reluctant to grant complete
Britain essentially wanted India to remain apart of the imperial defence and had hoped they would achieve that with self-governance by aligning both armies.
But the British still hoped that a self-governing India would remain
part of their system of 'imperial defence'. For this reason, Britain
was desperate to keep India (and its army) united. These hopes came to