I reckon it's due to the risking Bitcoin dominance. During the 2017 boom and drop we saw altcoins follow heavily based on the market, if BTC went up altcoins had to go up, and vice versa. That was when the market was fairly spread out between crypto-currencies. Nowadays, people are only putting money in Bitcoin and therefore Bitcoin dominance is at all time high, and the altcoin markets are almost a separate thing.
The pendulum is now shifting heavily in favor of Bitcoin as we can see the dominance of it eating the percentage that altcoins commanded months ago. I guess this is quite a telltale sign that people are getting tired of many projects on the world of cryptocurrency that are not actually performing well as expected. Now, coupled with all the scams, frauds and failures we all experienced, the sentiment and confidence plastered to many altcoins are eroding fast.
Looking long-term, this unbalanced situation is, of course, not good for alts and come the Bitcoin halving in 2020 there is a bigger possibility that they can still be left behind except maybe the top 5 or even the top 10 alts. As of now, we don't know when the market movement can be shifting back to alts but I know that it will only happen if we can inspire more trust and confidence with them.