Apache Hadoop was once considered one of the tools you should have as a Data Scientist. Around 2012 to 2014, it gained the popularity steeply among Data Scientists, and seemed to be considered one of the toolsets on the same boat as Python, R, and SQL.
But these days I have not heard its story. In fact, the Google Trend also agrees with me, that the popularity is peak at May, 2015.
Why has Hadoop failed to become one of the necessary tools as a Data Scientist, like Python or R?