Partly because Challenger was the first time something quite like that had happened in the American space program. The Apollo fire was on the ground during a training session, not even an actual launch, and by the time Columbia happened it was just a part of history that a space shuttle had already lown up once.

Then there's also the fact that the shuttles were still relatively new at the time of the Challenger disaster, and kids across the country were watching the launch live, partly due to a teacher being one of the astronauts, so it was kind of a publicity thing.

Columbia was still a tragedy and impacted many people greatly, but it just wasn't quite as unique a situation.