The author places الفاعل first in the list of nominatives. Beginning with the fāʿil is useful because it establishes attribution to a verb and a core sentence structure.
However, the eight categories do not share one identical construction: the fāʿil, nāʾib al-fāʿil, mubtadaʾ, and khabar have distinct roles; inna, kāna, and similar particles or verbs operate according to their own rules.