Why did Allah Almighty say in Surat Maryam, "من كان في المهد صبيا" (who was in the cradle a child) instead of "من هو في المهد صبيا" (who is in the cradle a child)?

Interpreters and linguists differed concerning the meaning of "kāna" (كان) in His (the Most High's) saying: "man kāna fī al-mahdi sabiyyan" (مَنْ كَانَ فِي الْمَهْدِ صَبِيّاً - who was in the cradle a child). Ibn Kathir was of the view that the meaning is: How can he speak, he who is in his cradle in his infancy? Al-Tabari said that "kāna" here is for completion (tamām) and is not the one that requires a predicate (khabar), likening it to His (the Most High's) saying: "hal kuntu illā basharan rasūlan" (هَلْ كُنْتُ إِلَّا بَشَراً رَسُولاً - Was I anything but a human messenger?), meaning: Am I anything but a human messenger? He also mentioned Zuhayr's saying: "wa qad kāna lawnu al-layli mithla al-arandaj" (وقد كان لون الليل مثل الأرندج - And the color of the night was like arandaj), meaning it became or existed. And it was said that "al-mahd" (المهد - cradle) here means his mother's lap.