Because the other 7 tribes had not yet asked for their land, Joshua chided them and told them to get going. They were to survey the remaining land, divide it into 7 parts and they would be given out to the tribes in lots. The first lot described went to children of Benjamin.
In other words:
Joshua had to tell the remaining tribes to stop procrastinating and get on with taking control of their lands.
That the lots will be decided by casting lots. That’s what you get for not taking the time to demand the land you want. (Joshua 18: 6)
Joshua 18:3 That Joshua has to chastise the remaining tribes to take their land. “How long are ye slack to go to possess the land?”
You snooze you loose.