Even though Nephi had been promised that what he would loose on earth would be loosed in heaven, Nephi does not presume to command God. He makes requests. He understands that these things are done through God’s power, and he maintains his humility, even though he has been given such a great promised.
Therefore, Nephi asks. Nephi notes that the people have repented, which was the purpose of the famine. It might have been a repentance of convenience, brought on by desperate times, but it at least put the people in a position where they were once again willing to listen to the word of the Lord.