They turned to Moshe and asked him to raise it. By subjugating themselves to him, by collectively deferring to him, they personified the ingredient most essential for hakomas ha'Mishkan, raising the Mishkan - unity. Moshe accepted, and succeeded in raising the Mishkan as a result of their collective achdus, unity. This is a thoughtful exposition regarding Chazal's critique of the Nesiim. We suggest that the lesson to be derived is that achdus is not a natural phenomenon.
People have to overcome inner challenges to act in harmony with others.
They have to work at achdus if it is to become a reality. We do not make committees for unity and expect it to just happen. One cannot pay lipservice to unity. One who is slothful, who takes an apathetic attitude towards life, will not succeed in sustaining harmonious relationships with others. This does not mean that the Nesiim were chas v'shalom, Heaven forbid, lazy in any way.