Lord Krishna is only supreme God in Bhagavatam, but Lord Narayana is supreme in the Vedas. Why? Is Lord Krishna only supreme God and not Lord Narayana?
Both names refer to the same Divine Person. In the Mahanarayana Upanishad 1.29 it says (Swami Vimalananda translator):
May we know Narayana. For that, may we meditate on Vasudeva. May Visnu impel us towards it.
and Vimalananda's commentary on this verse:
the Highest Person here is supplicated as Narayana, Vasudeva, and Visnu. Until and unless He impels the individual Soul, it cannot contemplate Him, and without contemplation on Him, His true nature cannot be understood. the etymological meanings of the three epithets, are given at length in the bhasyas on the Gita and Visnusahasranama. Bhattabhaskara's explanation of these words in the Visnugayatri is noteworthy. He derives Narayana thus: The term Nara, being derived from the root nr to lead, means the leader of all creation. Nara derived from the above word denotes his offsprings. Narayana therefore, is the effective cause of all creatures. Thus Narayana is Paramatman. He is called Vasudeva in his immanent aspect, i.e., dwelling in all creatures as Antaryamin. The term Visnu implies His all-pervasiveness.
"various are Thy names, O Lord, in each and every name Thy power resides..." - Sri Chaitanya