It is hardly surprising that AV was rejected but what we do not know is why.
There are five likely reasons:
1) It was associated with Nick Clegg.
2) It was too complicated for most.
3) Some LabCon supporters feared that it might be a step towards PR.4) Some prefer FPTP.
5) My suspicions are that there were many in the 'no' vote who were holding out for PR and did not want a poor substitute which might knock that off track.
(I spoiled my paper with a demand for PR.)