The Conservative Party break down into several different factions. Unfortunately for Cameron, because he had no majority at the 2010 General Election, he is forced to try to keep everyone on board, which includes perennial failures like IDS who monthly demonstrates he is not up to the job.
If Cameron had a good majority, lightweights such as IDS would be consigned to the fortnightly blue wheelie bin of industry. IDS is only there, and remains there, because Cameron is in a weak position .