It's not the difficulty per se, it's that it requires much, much more time, effort and coordination than anything else in the Trials Roulette pool, the rewards aren't nearly enough to make it worth the effort, and as a result people are starting to avoid Trials Roulette like the plague. Which causes its own problem in terms of dragging the experience level of people still doing it down, which makes it even harder for new people to get in to the damn thing in the first place, which means more people have more negative experiences and decide it's too hard and overtuned, and so on.