Gandhiji decided to launch nation wide satyagraha against Rowlatt Act in1919 because of the following reasons:
The success in satyagraha movements in regions like Champaran, Kheda had prepared the ground for satyagrah
The terms and conditions of Rowlatt act were very obnoxious, they were violative of civil rights of Indians.
Jallianwalabagh massacre was a tragedy and inhuman and barbarous
The Khilafat issue had brought in the opportunity of binding Hindus and Muslims. (Hope it helps. :) ... Plz send a thnx n rate it ! )