The word أَفْضَلَ is an ism tafḍīl (comparative), but its syntactic role here is the khabar of lā; hence it is accusative.
مِنْكَ answers “better than whom?” It complements the predicate, not the ism.
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Why Is Afḍala Accusative?
Ism of Mā and Lā Resembling Laysa; Khabar of Lā of Generic Negation
The word أَفْضَلَ is an ism tafḍīl (comparative), but its syntactic role here is the khabar of lā; hence it is accusative.
مِنْكَ answers “better than whom?” It complements the predicate, not the ism.