وَأَمَّا تَأْخِيرُهُ ... فَلِأَنَّ ذِكْرَ الْمُسْنَدِ إِلَيْهِ أَهَمُّ
As for delaying the predicate, it is because mentioning the subject is more important.
A normal assertion first establishes whom or what it concerns, and then supplies the information. In «Zayd qāʾim», the listener identifies Zayd before receiving the statement that he is standing.
The delayed predicate is not unimportant. The ordering follows the usual communicative sequence: introduce the topic, then convey the judgment, unless a specific rhetorical purpose recommends another arrangement.