Those who first heard the Good News did not receive that rest, because they did not believe. There are, then, others who are allowed to receive it.
This is shown by the fact that God sets another day, which is called "Today." Many years later he spoke of it through David in the scripture already quoted: "If you hear God's voice today, do not be stubborn."
If Joshua had given the people the rest that God had promised, God would not have spoken later about another day.