What were the conditions that were present in India in the 1920's that allowed Gandhi and Nehru to rise to prominence as nationalists?

Indian resentment towards British control was one of the major conditions that fueled the rise of nationalist leaders such as Gandhi and Nehru.
In the 1920s, many Indians were angry at how the British were treating them.  One source of discontent was the lack of acknowledgement of Indian sacrifice during World War I.  Indian servicemen fulfilled their duty in fighting for their parent nation, Britain.  However, after the war, Britain's actions failed to recognize such sacrifice. Britain showed dismissiveness and disdain towards the Indian demand for greater autonomy in taxation and political self-determination.  This began to fuel resentment and the ascension of nationalist leaders like Gandhi and Nehru. 
In the 1920s, British actions justified nationalism as a viable response.  The "Quit India" campaign intensified with events such as the Amritsar Massacre, when British forces opened fire on unarmed civilians. Leaders like Gandhi and Nehru began to call for demonstrations against British rule.  Gandhi was particularly effective with his insistence on nonviolent civil disobedience.  He made Indians believe that the struggle for independence was more spiritual than political.  Being able to make the call for nationalism a moral one cast the British as evil and unjust. Through Gandhi's example, Indians viewed opposing the British as an ethical imperative.  Nehru's focus was equally effective, but focused on the political aspect.  He helped to form the Indian National Congress. This political party's sole priority was the independence of India.  Gandhi's call for moral action and Nehru's political organization fueled their their rise as nationalist leaders in 1920s India.
https://www.nationalarchives.gov.uk/education/empire/g3/cs3/background.htm