It's because at the early of 2018 many projects are failed and some of the are scam. This is why investors are losing interest with altcoin and choosing bitcoin for long term investing
Many projects already failed and many projects will fail. Scam projects and scammers damaging the crypto word so much. Without them more secure investment portal can be build at crypto.
Addition to scam projects there are also death projects. Not even one single code in a year. More than 650 projects are in this situation.
So people are right for investing just BTC. By the way never forget to invest in strong projects and teams as BNB and ETH.