In general people are getting scared about huge amounts of scammers around in the crypto market. May be people are going back to conventional trading and investment platforms such as stocks, mutual funds etc..?
Another possibility is that most investors have lost most of their money already and have decided to never trade any market again, this is not so rare it is known that in other markets most newbie traders will lose all their capital in less than a year, why this market should be any different? Especially when the amount of scams in this market is many times higher than what you could find in the stock market, and until we can bring more investors then altcoins are going to remain in their place.