I don't know why Liverpool can't perform well in all the other tournaments except the Premier League. Before that, I clearly remember their performance in the European League where they were the strongest team but they couldn't qualify for the semi-finals of the European League that season.
I mean, that's just the nature of cup competitions. You have to perform at every game, and consistency across 20 games doesn't necessarily translate well to knockout rounds performance. They did perform well on the group stages, winning nearly every game iirc. On the other hand, other teams are simply getting better, like PSG. It's not like Liverpool suddenly plays like Sunderland or something.