1. The presence of British in India was not small
East India Company had a sizable Army in India. By 1806, they had about 154,000 armed men and far more than most armies at that time.
By 1857 mutiny, this army more than doubled:
Besides their army in India, Company & later Britain had Navies in the Indian ocean from Malaysia, Australia to South Africa. These navies helped bolster the administration and Britain could quickly move the troops in case of trouble. The creation of railway system further strengthened the administration by easing troop movement within India. In a few decades, Mahatma would turn the same advantage against them, as he used the radio, newspapers and the railway system to connect common Indians.
2. India was a divided nation then
After the disintegration of Mughal empire there was nothing connecting India. That means, although there were uprisings in various parts of the country, they were all random and uncoordinated. For instance, some group in Vellore would rebel. The company would send 2000 troops and they will be squashed. A month later, another group would rebel in Bombay and the company would send the 2000 men there. The chain would go on.By brutally squashing the uncoordinated rebellion, they sent a strong message. Unless, a big chunk of India made a coordinated attack, the company could just keep sending a few thousand strong warriors to squash rebelling everywhere.
This is where Mahatma's genius foiled British plan. Instead of the Maratis, Bengalis and Tamils attacking the British rule in a sporadic way, he made them act coordinated at the same time. When whole of India launched a coordinated attack, there was little the British army could do. He also removed the fear that was keeping the commoners chained.
3. Help from Princely states
The British didn't rule all of India. A big chunk of them was ruled by Indian kings. These local kings were all loyal to the British (those not loyal were elminated) and actively helped squashing rebellions in the British controlled regions close to their state. For instance, the British would take Hyderabad's and Mysore's help if they need to fight a rebellion in Madras. Only the pink territories were ruled by the British and they were all adjacent to the princely states.
4. Winning the elites
Britain won over Indian elites (especially Hindu elites) and these elites helped give legitimacy to the empire. For these Indian elites, British rule was a welcome break from a long period of oppression under Islamic rule and also provided a chance to reform Indian society. From Raja Ram Mohan Roy to Dhadhabhai Naoroji (the first Asian MP in Britain) to Motilal Nehru, Indian elites of that time went along with the British rule. People like Sir Surendranath Banerjee and Netaji Bose served as Civil Servants.These Indian elites helped pacify the masses. Mahatma simply entered and cut the cord. Indian elites came out flocking. That pulled the legitimacy from under their feet and now they were made to look like real invaders (previously they were a part of a "reformatory" system that included the Indian elites).
5. Indian Masses don't Revolt (unless our religion is touched)
Indians are known for extreme tolerance and we never really rebel, as long as our daily life is not impeded. This is the reason why we could be ruled by so many invaders of every stripe. The only time we revolt are when our religious practice is affected (such as in 1857 when our troops were made to touch beef/pork coated cartridges).This is the general pattern throughout our history. In 185 BC Pushyamitra Sunga led a coup in Maurya empire due to the imposition of Buddhism. In 18th century AD, our kings revolted against Aurangzeb's rule when he started imposing Islam on India. Aurangzeb's religious zeal helped end the Mughal empire. Religion is the biggest unifying thing in India & it is quite easy to turn the masses with religion.
Unlike the French, Dutch and Portuguese, British were careful not to interfere in India's religious practices. They were smart. Thus, Indians just let them be. If you don't mess with India's religious practices, your politics is generally safe. Mahatma was quite smart in bringing religion into the freedom struggle. Without bringing religion, you cannot bring the Indian masses out.
6. Use of existing systems & hierarchies
Britain kept existing hierarchies intact and used them effectively. Thus, the Zamindari system of Mughal time was actively utilized to collect tax from the peasants.Indians of 18th and 19th centuries were quite closed to change. Thus, retaining the old systems meant that Indians had lesser motivations to rebel. However, if it were any other colonial power, such as the Spanish, Indians would have been hard to rule. The Spanish messed too much in the regions they ruled.
