The transition between the generation that left Egypt and the one entering the Promised Land required a profound renewal of their covenant. Over the years of wandering, a significant spiritual and practical gap had formed between the older and younger members of the nation.
The primary approach among commentators is that the entire generation that departed from Egypt had already been circumcised. Just before the Exodus, Moses instructed the people to fulfill this commandment so they could properly participate in the Passover sacrifice [ביאור שטיינזלץ, צאינה וראינה]. Ultimately, the adult men of war from that era died in the desert because of their sins. The only individuals from that original group who survived to enter the land were members of the tribe of Levi and those who were under twenty years old when they left Egypt [מלבי״ם, אלשיך].
In contrast, the new generation born during the long journey through the wilderness remained uncircumcised. The responsibility for this lay with their fathers, who did not perform the procedure on their sons [מלבי״ם]. This delay was caused by the harsh realities of their environment. The desert climate lacked the northern winds necessary to help heal the physical wound [רש״י], and the constant, difficult travel made safe recovery impossible [ביאור שטיינזלץ]. Because of these challenging conditions, the practice was paused. Once they crossed into the land and Joshua finally circumcised them, the people had to remain in their camp until they were completely healed [צאינה וראינה].
Looking deeper into Joshua's actions, there were actually two distinct types of correction taking place within the nation. The first group consisted of those who were young when leaving Egypt. They had undergone basic circumcision but lacked the final step of the procedure, which Joshua now completed. The second group was made up of those born in the desert who had never been circumcised at all. For these individuals, Joshua performed the entire procedure from start to finish [אלשיך].