The reason they don't put any secure door on the flightdeck is that as a 19 seater, it wouldn't usually qualify to carry cabin crew, so the only folk who can look after the pax is the pilots, thus they need to be able to look over their shoulder or intervene in a medical emergency, etc.
The J41 is a 29 seater so does need CC, hence it's safe to seal the flightcrew off. I think.....