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
This is one of the reasons for it, and probably, even the main one. However, there is another reason for it. Altcoins do not show explosive growth because a period of accumulation has started, and it is supposed to last until 2020. After that, the best-rated (top-100) altcoins will go to the Moon, and other tokens will also add. Now, more people are concentrated on deals with Bitcoin as the most reliable crypto.