“ That is why the children of Israel could not stand before their enemies; they turn their backs before their enemies because they are become devoted. I will not be with you any more until you remove the devoted thing from among you.”
Because the devoted thing was among them they too were devoted to destruction. Thus they received no assistance against their enemies. Indeed that was why they had turned their backs on them. The only way to change the situation was to remove the devoted thing from the camp, and this would include all who were directly affected by it. Achan had brought his family into his sin. Some of them no doubt knew about it but did nothing. But all would suffer for his sin. We need to remember that in the end our sins and attitudes directly affect others.
“I will not be with you any more until you remove the devoted thing from among you.” ‘You' is in the plural. Here YHWH changes his approach to speak as though directly to the people, both to make the words more vivid and to remove any suggestion that Joshua is himself in view. Such sudden changes in person occur fairly regularly elsewhere.