Why do the propagators of Hindutva or communism try to revise the history of India?
They do because the history is way too biased. And this was deliberately done in order to keep rival ideologies at bay. You might be astonished to know that most of the historians were from a single university and the university is notoriously famous for its left leaning. Some names are Romila Thapar, Bipin Chandra, Satish Chandra, Irfan Habib etc. There has been two instances when government had tried to remove this bias (actually garbage) from history books. One was during Janta Dal time and second was Atal ji's time. Both time these so called historians (rather Marxist academicians) opposed tooth and nails. However if you carefully look at the debate academically, the revision was quite legitimate. In our curriculum we are fed too much on Mughal period which was perhaps worst period as no development of any sort can be reported. However if you go through history books you would wonder if we are still under a Mughal rule. There were many large and far more glorious and successful periods in Indian history which has been avoided. And ofcourse being a north Indian, we are not taught Maurya adequately leave Chola aside. Having said that I am happy that with the rise of technology and social media some has exposed to the hypocrisy & lies and few have realized the somewhat correct or at least "better" version. This partially answers a related question as why Indian youth has become aggressive because they have access to truth and can make out the blatant lie. Time has gone when PhD is called Babasahab and mere graduate is referred as Pandit.Lastly I repeat a tweet which I read when Bipin Chandra dies few days back. I am modifying it a little bit to suit the purpose."May the History treats the Historians same way as they have treated it."