Many people assume it was because of poorly-functioning O rings on the booster rocket.
However, those O rings didn't send that ship up on a cold winter's morn. People did, and those people drew their most critical information from two simple charts, screened by an overhead projector. The graphs displayed tiny pictures of each shuttle booster, lined up in chronological order, showing launch temperatures and any O ring damage. They looked like so many crayons in a box, and when the engineers and managers finished looking at them, they didn't know any more than they had before. The launch was made and seven people died.