A bodhisattva has achieved the goal of enlightenment through his life. He could therefore enter nirvana after his death.
But he has created a vow that he will renounce nirvana for as long as he lives. Compassion for all living beings becomes more important than the finality of one's own salvation.
Having attained enlightenment, he therefore does not enter nirvana. Instead, he continues to act as an active helper for the liberation of all beings suffering in the cycle of rebirths. Until all beings - including not only humans, but also animals and gods - have been led to enlightenment, a bodhisattva does not accept the nirvana he has earned.