Keeping public attention on the space program (even if negative) is one possibility since it was declining in public interest.

Another possibility is that the shuttle launches never actually had the "astronauts" on board and they were sending up dummy shuttles (then the real shuttle would be flown up on a 747 to land later with the crew). On this launch, the dummy shuttle exploded due to a genuine technical problem, so they had to play it off as though everyone died.