Straight are the paths of the LORD, in them the just walk, but sinners stumble in them.
Usually, I think, sinners are said to be those who aren't on the paths of the LORD at all, as for example when Jesus contrasts the broad road that leads to destruction and the constricted road that leads to life.
We might infer from Hosea one reason sinners usually avoid the paths of the LORD: When they're on them, they stumble. Walking along the constricted road is painful and humiliating. Who wants that?
No one, really, but if you have an idea of where the path leads, then you're willing to put up with falling seven times, because you can rise again and not stumble to ruin.