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.
Krishna Lilas - The Nectarian Pastimes of the Sweet Lord