The reason why was the war of 1857 hailed as the first war of independence is that, It was the first significant and massive rebellion against the rule of the East India Company during the 19th century. Infact there were many such rebellions but the scale of what happened starting from Meerut on 10 May 1857, was much more than others.
This is the reason its celebrated as our first war of independence in India.
So for the question as to
Why was the war of 1857 hailed as the first war of independence
In short its because it was first major revolt against the Britishers.