From what I have read on the net since the disaster it now appears to me that the reason the accident took place at all was simple – NASA cutbacks. Since Bush came to office cutbacks have had a serious effect safety, indeed Bush received a report not too long ago warning of a potential disaster due to lack of funds. This article by Richard Feynman also demonstrates a flaw in NASAs thinking.
As well as that the reusable launch vehicle intended for the ISS was cancelled due to budget overruns.
Bush is keen to spend up to $100 billion on war with Iraq, but cannot spend $5 billion to revamp NASA.