Coal is very friable (breaks easily) so you do not need the force a cone crusher can supply. Also a cone will create more fines than a roll crusher. Another factor being that both roll crushers and impactors take up less height than a cone so the overall height can be less.
Clarifiion: Crushing makes the top surface of a coal furnish due to which it becomes easy to handle (by reducing its overall top size). Screening is used to group process particles into ranges by size. By appropriate sizing, transportation of coal becomes easier.