A scandalous decision made early in the life of the aircraft by Their Airships even though a rear crew ejection system had actually been designed. The reason - cost. It seems that lives were cheaper in those days..
Even when they converted to the low level role, the rear crew were supposed to bail out through the bottom hatch, with the very real possibility of the 'seat inflators' trapping them in the process. We regularly used to practise 'Escape Trainer' drills, the rear crew (Nav Plotter, Nav Radar and AEO) had a complicated drill to go through which involved the right person blowing open the hatch, the seats being moved, cushions inflating to assist seat egress and abandonment taking place in the correct sequence. Of the various Vulcan abandonments, rarely was this fully successful, even at altitude.... With the undercarriage down, the noseleg was immediately behind the lower hatch and the rear crew were supposed to grab a lower hatch jack and roll themselves away from the noseleg. Even a racing-snake PTI in a wind tunnel found that pretty difficult, the chances of the average cuddly rear crew person in goon suit and LSJ being able to do it were infinitessimal. We used to fly the Vulcan in the circuit with the undercarriage down all the time, so the natural enthusiasm for the rear crew to enjoy protracted periods of pilot playtime was even less in the Vulcan than it was in most aircraft!