There was considerable doubt over the verdicts in a couple of hangings which had occurred in the previous year or two. This was enough to lead parliament to vote against it. The general population was very much in favour of keeping the death penalty.
Shortly after the death penalty was abolished, Myra Hindley and Ian Brady were tried and convicted for the sexual torture and murder of a number of small children. It is inconceivable that any government could have got away with abolishing the death penalty if those crimes had been uncovered before the vote.