The Biblical strict interpretation answer to that question would be that the Hebrews refused to follow God's lead. Relatively soon after the nation escaped from Egypt, spies were sent into the land of Canaan to evaluate the land. Caleb and Joshua were among the spies and, upon their return, encouraged Moses and the tribes to take action to enter the land immediately. The other spies, however, reported seeing well-armed giants and strongly defended cities in Canaan. The people refused to attack at that time; God decreed that all those who had refused to obey his direction and enter at that time would die in the desert and that a new generation would be the one to cross into the new land.
Those who interpret the history less literally would say that the tribes spent the 40 years fighting with other tribes throughout the region as they traveled, searched for food for their flocks and built the discipline and organization needed to be able to conquer the Canaanites.