In the poem, "The Divine Image," why does Blake say "all pray in their distress"? In what way are the people distressed?
When you ask why Blake writes "All pray in their distress" you are really asking a question no one can answer. We cannot speak for a writer. No one can say with certainty the motivations of any writer, whether the writing is contemporary or 200 years old. We can only study the text and must speak of results, functions, effects. We cannot pretend to speak about what was in a writer's mind.
That said, we can judge from the text that Blake is speaking in general terms here. The "people" of the poem are not in distress for any particular reason, that is not the point. The point is that when they are distressed, they pray to "...Mercy Pity Peace and Love."
Furthermore, if you really want to get a handle on this poem, you should study its contrary poem in Songs of Experience: "The Human Abstract."
Blake says that all people "pray in their distress" because he believes that people tend to pray most when they are in distress -- when they need help of some sort. We have a saying "there are no athiests in foxholes" meaning sort of the same thing -- that when people's lives are in danger (as in a war) they all pray.
Blake never says in the poem why people are distressed. The poem is really about what attributes God and people have. But distress is part of the human condition. We all exprience distress when we are sick, when we have troubles, when people we love die. That's just how it is when you are human.