To be fair, there were specific temperature ranges in which the shuttle was supposed to be capable of launching, and the temperature range did not go as low as you might expect because the Shuttle was going to launch in Florida (and, maybe someday, California) - iirc, the minimum temperature was 40 def F or something like that.

Of course, the Thiokol engineers weren't sure that 40 was sufficient (they were worried about anything below 52 degrees), but in defense of the people that chose the o-ring material, Challenger launched outside of the design spec for the Space Shuttle.