I am also confused what causes the price of crypto decline so deep, maybe many investors withdraw their funds invested in the crypto or can also leverage the influence of the world economy is sluggish
It has been a series of bad news.
-> First it started with Korea and fears of legislation
-> then the Mt. Gox sell off occurred and some people blamed it as the reason for the depression, although that is questionable: https://cryptoslate.com/no-mt-gox-sell-off-not-blame-market-dip/ - in any case it caused lots of FUD
-> then serval large companies banned ads on crypto: https://en.bitcoinwiki.org/wiki/Cryptocurrency_Ads_Ban
After all, nothing really bad happened, just in December/January prices were too high compared to the technological development so a correction needed to occur. Give it 1-2 months and prices will stabilize/rise again!