Shuttle got very lucky. On the first flight, STS-1, an overpressure caused by the ignition of the SRBs forced the orbiter's body flap into an extreme angle which could have destroyed the hydraulic system controlling it. Had John Young know this had happened, he and Robert Crippen would have ejected, which would have destroyed the orbiter on its first flight.
There were only 2 ejection seats, enough for the crew of test flights but not the larger crew of operational flights.
The seats were only installed in Enterprise (the prototype, used only for suborbital tests) and Columbia (only enabled for STS-1 through STS-4 test flights, disabled for STS-5 the first operational flight)
The seats would only work at low altitude and speed (I've seen differing numbers cited). For the Challenger disaster they would've theoretically been useful (ignoring all the other factors), but they would've been useless for Columbia due to speed.
And it's not clear ejection would have actually been successful with the SRBs still active and right there.
As o11c mentioned, they only existed for the first few flights, the ones that only had two crew. It wasn't possible to have the election seats with the full shuttle crew so they were removed.
The ejection seats were essentially the same as those used in the SR-71, so they were survivable at shockingly high speeds and altitudes.
You could eject from the Space Shuttle? At what speed and altitude? What was the mechanism?