Placing the subject first can prepare the hearer to attach what follows to the correct referent. The text treats this as a way of making the predicate settle more firmly in the listener's mind—tamkīn al-khabar (تمكين الخبر).
If the audience is waiting to hear about Zayd, introducing him before the decisive news keeps the person salient. Then the subsequent assertion can be more readily assigned and retained. An event-first arrangement may have a different focus.
This is not simply a function of length. It arises from the relation between the listener's anticipated topic and the order in which the speaker supplies it.