Why did Achan's sin bring judgment on the entire nation?.. Jos7

Although it was one men's failure. God saw it as national disobedience to a national law. God needed the entire nation to be committed to the job they had agreed to--conquer the land. Thus, when one person failed, everyone failed. If Achan's sin went unpunished, unlimited looting could break out. The nation as a whole had to take responsibility for preventing this undisciplined disobedience.

Achan's sin wasnt merely his keeping some of the captured goods (God allowed it in some cases), but his disobeying God's explicit command to destroy everything that connected with Jerico. Achan's sin was indifference to the evil and idolatry of the city, not just a desire for money and clothes. God would not protect Israel's army again until the sin was removed and the army returned to obeying HIM without reservation. 

God is not content with our doing what is right some of the time. We are under HIS orders to eliminate any thoughts, practices, or possessions that hinder our devotion to HIM.