Bitcoin Cash is just another alt. It has no more or less bearing on things than any other major coin. Almost all of them regularly post higher gains than Bitcoin percentage wise because they're all smaller.

2018 is nowhere near the worst year yet.

The one and only reason for these lulls is that giant bubble that you might have noticed. Bubbles pop when enough people realise they stupidly overpaid for something and run for the door. It takes a long time to rebuild confidence in valuations once it's been torched like that.