Various reasons, by just looking at the screenshot you posted noone can diagnose what happened. Did you change anything on the website/robots.txt/.htaccess? Exact day the traffic started dropping? Any message in the SC? Any errors suddenly popping up in SC? All fine with your hosting? Is you site responsive/mobile friendly? Do you have different versions for mobile/desktop? Did you add any new links recently? Does your backlink profile look clean overall? Any known onpage problems (loads of duplicate content, thin affiliate/autogenerated site)?
I could continue this list of questions for another 20 questions, basically: The less data you give us, the less likely someone will be able to tell you a reason and people start the useless "My crystal ball says...." game to increase postcount.