As already said, NASA doesn't want another Challenger.
When Challenger happened, NASA was in a rush to put Christa McAuliffe in space because of the much hyped Teacher-In-Space contest. The launch though was pushed back from it's initial date a couple of times from a previous launch, and some glitches.
Then on the morning of the launch, it was around 30F. This little bit of change killed all 7 people, because in the boosters they had rubber o-rings that would swell on takeoff. But it had never been cleared for 30F, because what the commons sense was... if the o-rings WERE too cold, they wouldn't stop the gasses from escaping... which doomed the flight.
Around 70 seconds into the flight, all the hot gas started to burn a hole in the adjacent fuel tank. While the Challenger didn't explode into a fireball, the orbiter was ripped apart from the destruction of the tank's gasses finding a weakpoint and destroying it's tank.
Rockets are full of complex shyt, and one error; with Challenger and Columbia, can cause failure quickly because they are fragile.