قَدَّمَ الْفَصَاحَةَ عَلَى الْبَلَاغَةِ لِتَوَقُّفِ مَعْرِفَةِ الْبَلَاغَةِ عَلَى مَعْرِفَةِ الْفَصَاحَةِ
Fasaha comes before balagha because understanding balagha depends on understanding fasaha.
The fuller definition of balagha will require reference to fasaha. If the student does not yet know what it means for discourse to be fasih, that prerequisite will remain obscure in the definition of baligh discourse.
The order is thus conceptual: explain the foundational notion before using it to explain the more comprehensive rhetorical evaluation.