Just a guess, so take it for what it’s worth:
The Apollo missions had drastically altered expectations about the ease of taking a jaunt to the Moon. Space seemed easily navigable and conquerable. Star Trek and Star Wars were at the front of lots of people’s minds.
They needed space travel to be “difficult” again, since it had been over a decade since the last “moon landing.” They needed a high profile tragedy, and they needed some “regular” people on board, to boot.
The “Challenger” plan was likely in place as early as 1982 or 1983, before the initial launch of the shuttle. Even the name, itself, implies the difficulty, or the “challenge” of space travel. Blowing it up with a fake crew on board, and crisis actors on the ground, would make a great headline and they could milk that for some time.
They also could argue the need for more funding due to greater rigors of space travel, and deflect any questions about getting back to the Moon for some time.
It likely also distracted from the 1986 assault weapon ban.
And, it worked.
Don't forget the 86 AMNESTY as well.
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