The reason why the Buddha has to do it all over again, is because he has to earn merit again. He’s still a Buddha, of course. Meaning that all the merit that he earned, he had to use to give it to these people, in order for them to rise up to become Arhat, all liberated. The merit in Heaven and the merit here is different. Of course you have merit from Heaven, you are a Buddha. He was a Buddha long long time ago. But if he wants to come back to this world again, then he has to earn it again. He has to sow affinity with other beings. If you have no affinity with human beings, or animals, you cannot save them.