There is no central leader to unify them, to guide them, and shepherd them as a parent would do. I've heard some of you question why God didn't continue to provide them with a leader, or why Joshua didn't train a successor. Even if they had appointed themselves a leader, it is likely they would have rebelled against that leader, much as many college age students tend to rebel against society. It was time for them to learn if their past obedience had been internalized by the nation, or if it was only there superficially, to escape punishment and placate authorities. Let them see that independence, without an authority figure demanding that you do the right thing.
Independence requires mature judgements.
Since you reap what you sow, these Israelites pay the consequences for their immature disobedience and rejection of their earlier teaching. But, from God's wisdom and Mercy, He raises up Judges to save them from their servitude to sin and invaders. These judges, as a whole, are not the best examples of godliness, but they are submissive enough to be used by God. These judges were not judges in a judicial, legal sense, but from the sense that they were involved in God's judgement on the invaders, and God's judging that the Israelites were humbled and repentant enough to receive His intervention. There is a great variety of types chosen to lead these individual tribes as they come under duress.