A grim story tends to leave a reader with a clear sense of a problem but no obvious next move, which can produce a kind of passive resignation: the situation is bad, and there appears to be nothing in particular for the reader to do about it beyond simply feeling worse.
A hopeful story, done well, includes the difficulty just as honestly but pairs it with a visible response, someone doing something about it, which gives the reader a mental foothold: a sense that action is possible, even if their own contribution would necessarily be small in comparison to the scale of the problem.
The difference is not about softening bad news, since a genuinely hopeful story does not pretend the problem is smaller than it is, but about completing the account, showing not just what is wrong but what a response to it can actually look like in practice.