After the first few shuttle flights, the ejection seats were removed and the crews wore little more than a jump suit and a helment similar to those in their crew photo. This continued until the Challenger explosion. In the accident investigation, it was determined that the crew survived the initial explosion while the now depressurized crew compartment continued on an upward trajectory. Some of the evidence came from emergency oxygen packs that had to be manually activated However, once a human gets much above 40,000 feet, simply providing oxygen under pressure isn't enough. Above that altitude, you need external pressure on the body to breathe - either from a pressure suit or from a pressurized cabin.

When Shuttle flights resumed, the crews wore much more rugged pressure suits as seen in the Columbia crew photo. The reason was that they now had a limited bailout capability (sliding down a pole to get clear of the wings) and the suit expanded the bailout window somewhat and gave them a better chance of surviving. However, those suits weren't nearly strong enough to survive the breakup at high Mach during the Columbia reentry. Likely nothing short of an ejection capsule like the ones developed for the B-58 would've allowed them to survive. I do know of one instance where an SR-71 pilot survived a breakup at Mach 3.18 due to his pressure suit.