The Israelites were sent back to the desert for 40 years primarily because they listened to the spies who reported about the giants in Canaan and expressed fear about being able to conquer the land. After sending spies to scout the land of Canaan, the majority report from the spies was negative, claiming that the inhabitants were powerful and that the Israelites could not succeed in taking the land. This lack of trust in God’s promise and fearful reaction led to their inability to enter Canaan, resulting in God decreeing that they would wander in the desert for 40 years until that generation passed away.
So, the correct answer would be:
because they listened to the spies when they insisted that the giants who inhabited Canaan could not be defeated.