Decision on production a cartridge with a case with a paper tube may seem strange at first sight since advantages of the plastic case are obvious and acknowledged by time and practice.
Modern paper case is damp-proof enough as it is covered with special varnish. And possibility of multiple usage of the plastic case is rather relative because in general cases are used only once.
The cartridge has been designed primarily to ensure stable ballistic properties of shot at sub-zero temperature at winter hunting. Only cartridge with a paper tube case ensures stable shot in winter especially at -20oС and below.