Because the size of bomb bay needed to accommodate the Blue Danube bomb would have precluded this. Also, the need to pressurise the crew compartment meant that the smallest possible space was essential - don't forget that the aircraft was first designed around 60 years ago!
Rear crew ejector seats were entirely possible; however, as the Vulcan was due to be withdrawn from service with the advent of TSR2...F-111...AFVG etc. etc.
To reduce wear and tear on hydraulic components, in the Vulcan the landing gear was left down for visual circuit flying. The chances of the average rear crew member being able to escape with the nose leg extended were close to zero, so extended circuit work was highly unpopular with many rear crew....