In a nutshell, the challenge is how to provide an initial off-road cycling experience for beginners that increases the likelihood that they’ll want to continue. Beginner level cross-country trails (IMBA’s Green level) are often constructed in a way that’s too challenging for those who’ve never ridden anything but pavement and they quit.
It’s difficult to know which types of terrain these true beginners can handle and which they’re going to struggle with or freak out over. So trail builders make a guess, aiming for whatever they think the middle ground is (‘Goldilocks design’, ie, not too hard, not too easy) and hope that a majority enjoy it.
But then a pure beginner rides it, gets 100 yards in and they freak out over a slight downhill turn and end up being miserable the rest of the way because the trail has a lot of those. Or they’re tense every time the trail goes close to trees because they’ve never had to give much thought about the width of their handlebars. Or they’re anxious because they’re worried they won’t make it up a slight incline and fall over or backwards. Or they constantly worry that they’ll run into another rider because the trail is two-way.
And they decide that mountain biking is not for them.