«أَمَّا كَمَالُ الِانْقِطَاعِ فَلِاخْتِلَافِهِمَا خَبَرًا وَإِنْشَاءً، لَفْظًا وَمَعْنًى»
One form of complete disjunction is that the two clauses differ as declaration and non-declaration in both wording and function.
A report states a proposition; a command, question, or supplication performs a different speech act. When the speaker does not intend to bind these acts into a coordinated construction, adjacency is not a reason to add wāw.
For example, “He has arrived. Welcome him” contains a report followed by an instruction. They are related in subject matter, but they are not two parallel assertions of the same kind.