You can always say it's a coincidence, but is there particular need to have a helical capsid? Do they need their RNA to be more protected than +ssRNA? Is it better for RdRp attachment?
This paper suggests it is mostly for RNA synthesis https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3993539/:
For negative-strand RNA viruses (NSVs), eight families have been recognized by the International Committee on Taxonomy of Viruses (ICTV). The unique feature that distinguishes NSVs from the rest of the virosphere is that the nucleocapsid, instead of the naked genome, is used as the template for viral nucleotide synthesis. It is indubitable that the assembly of the NSV nucleocapsid is related to the unique mechanism of its viral RNA synthesis.
Also this one: https://www.sciencedirect.com/science/article/pii/S0042682215001543. But I don't know if anyone has tested inserting naked -ssRNA with RdRp and see if it does something.