They had swapped around the main engines so that the three highest-performance (capable of highest thrust) ones in NASA's stock were on the Challenger orbiter for that mission. They launched in shit weather (cold and severe wind shear), and had been warned that the launch was risky. The Challenger disaster was a severe setback for military projects which relied on the Shuttle to launch heavy payloads.
Challenger may have been destroyed by something other than sabotage, but it was a perfect target for the entities that didn't want effective missile defense deployed to sabotage.