The decline's reasons were as follows
(i) There was a lack of effective organization.
(ii) Communal riots were witnessed during 1917-18.
(iii) The Moderates who had joined the Congress after Annie Besant's arrest were pacified by talk of reforms (contained in Montagu's statement of August 1917 which held self-government long-term goal of the British rule in India) and Besant's release.
(iv) Talk of passive resistance by the Extremists kept the Moderates away from an activity from September 1918 onwards.
(v) The Montagu-Chelmsford reforms which became known in July 1918 further divided the nationalist ranks
(vi) Tilak had to go abroad (September 1918) in connection with a case while Annie Besant vacillated on her response to the reforms and passive resistance techniques.
With Besant unable to give a positive lead and Tilak away in England, the movement was left leaderless.