Only the pilot and copilot had ejection seats, the rest of the crew couldn't also have ejection seats (I'm not sure if it was specifically weight, volume, or orientation/configuration that was the limiting factor. I think maybe a combination of all three.) STS-1 through STS-4 (all using Columbia) only had a pilot and copilot, no other crew. For STS-5 and STS-9 when there was more crew, they flew with the ejection seats disabled. After STS-9 the seats were removed.
I don't have a source saying it specifically, but I think they weren't keen on operating a shuttle where two of seven crew could survive but the other five had no chance. It must have also saved weight and space too.
I don't have a source saying it specifically, but I think they weren't keen on operating a shuttle where two of seven crew could survive but the other five had no chance. It must have also saved weight and space too.