I want know the exact reason for that
You don't know, i don't know and experts also don't know the exact reason. People are looking for causes but there is no unified cause. Hold it and don't need to know the cause.
In summary, no one knows. As far as I am concerned, a lot of reasons could have caused the downtrend, but I am pretty sure a lot of it was totally as a result of manipulation of the price upward.
Nothing was meant to drive demand the way it did last year, no serious news to have caused such huge adoption, new people just simply started FOMOing because of all the media wanted them to know about how people are getting rich and in that case, they wanted to get rich fast as well. Unfortunately, panic set in when the whales started their course downward for a lot of the weak hands. Every other explanation is just people trying to find a reason but in reality this was bound to happen.