Brahmā addressed Lord Kṛṣṇa as Acyuta because the Lord is never forgetful of a little service rendered by His devotee. He is so kind and affectionate towards His devotees that a little service by them is accepted by Him as a great deal. Brahmā has certainly rendered much service to the Lord. As the supreme personality in charge of this particular universe, he is, without a doubt, a faithful servant of Kṛṣṇa; therefore he could appease Kṛṣṇa.
"You are the original lamp. Other incarnations may possess the same candle power as the original lamp but the original lamp is the beginning of all light." ~Lord Brahma, in "Prayers Offered by Lord Brahma to Lord Krishna" ..*