Volatility decreased as more and more institutional money moved in. Be careful what you wish for. Crypto bros welcomed when HFs started to invest in crypto, but it also caused BTC just being another boring (and eventually lower and lower beta) tech stock. This is the fundamental explanation.
The technical explanation could be that arbing between the exchanges got so good that even a whale dumping or pumping can not move the price that much (thus volatility died) and now the major force for price action is HFs buying or dumping.
The technical explanation could be that arbing between the exchanges got so good that even a whale dumping or pumping can not move the price that much (thus volatility died) and now the major force for price action is HFs buying or dumping.