Challenger was:
1) Using a new, poorly tested putty between segments on the SRBs ... because Save da Erf.
2) Launched significantly outside weather parameters. The ENGINEERS at Morton Thiokol said "Don't Launch". The MANAGERS at NASA said "Go for Launch", because they wanted to look good.
The Shuttles had been burning through O-rings prior to Challenger. They were plenty aware of the problem. The only reason it hadn’t bit them prior to that is that the burn through had never impinged on the LOX tank.