وَإِنَّمَا قَسَمَ كُلًّا مِنَ الْفَصَاحَةِ وَالْبَلَاغَةِ أَوَّلًا
The author first divides each of fasaha and balagha into kinds.
لِتَعَذُّرِ جَمْعِ الْمَعَانِي الْمُخْتَلِفَةِ الْغَيْرِ الْمُشْتَرِكَةِ فِي أَمْرٍ يَعُمُّهَا فِي تَعْرِيفٍ وَاحِدٍ
Because meanings without an encompassing shared feature cannot be combined within one adequate definition.
Fasih word, fasih discourse, and fasih speaker are related, but their full conditions are not identical. The same is true of the two subjects of balagha. Classifying first, then giving an appropriate definition of each class, avoids imposing one set of conditions on all.