There are obvious reasons why Modi is (CAN) not putting Congress leaders behind bars in corruption cases.
This is not the job of a PM. Judiciary is an independent arm of the system and it takes its own course.
Indian society being corrupt itself on various counts, starts connecting itself with the corrupt, especially the Congress and with the passage of time, begin sympathizing those.
If Modi begins doing this, all the media that itself is sold and corrupt, start shouting that Modi is doing Vendetta Politics and Witch-hunting the opposition.
All the cases related to Congress Scams are in the courts already. Because the cases were filed during the Congress Raj, investigated by the Congress-Parrot CBI, important files and papers were shown lost or damaged in fire by the Congress sympathizers and corrupt bureaucrats and ministries, vital information and evidences destroyed by the babus in connivance with their masters and money exchanged hands to keep shut up. Because in scams and money laundering, only documents are the evidences, it is very easy for those in power to destroy the documents or misplace them.
Only the government has changed, NOT the babus i.e. the bureaucrats. Unfortunately most of the bureaucrats being corrupt and belonging to Congress era, are still loyal to their previous corrupt masters. Keep in mind, not all the bureaucrats can be transferred nor there are many like Ashok Khemka.
There are some corrupt people in BJP also, who would certainly not like the idea of a clean government.
Only Modi or five or six of his good ministers can’t eradicate 70-year old corruption from India in three years time.
Most of the judiciary is also corrupt especially at the lower level.
Here, though not related to corruption, I would like to give one very important example establishing how short is the public memory. In 1984, over 3000 Sikhs were burnt alive on the streets of Delhi by Congress Karyakartas under the supervision of
Congress leaders like Jagdish Tytler, Sajjan Kumar, HKL Bhagat, Dharam Dass Shastri etc. All the Sikh people world over had been dead against Congress. And now in Punjab, Congress is back into power. This is the right example of corrupt values the Indian society bears.