AV was confusing. To those who knew what it meant, there were no obvious benefits, so why bother. To those who didn't know, the difference could only be explained by mathematics and unintuitive use cases, and no-one could make a compelling argument about why it was better, so why bother.
The argument for PR is far clearer. In the last general election (FPTP), Conservatives got 42% of the vote but won 50% seats; Labour 40% of the vote but 41% of seats; Lib Dem 7% of the vote but 2% of seats; SNP 3% of vote but 5% of seats; etc. THAT intuitively doesn't seem fair or democratic. PR seeks to reflect % of votes with % of elected seats, which does seem intuitively fair and democratic.
The argument for PR is far clearer. In the last general election (FPTP), Conservatives got 42% of the vote but won 50% seats; Labour 40% of the vote but 41% of seats; Lib Dem 7% of the vote but 2% of seats; SNP 3% of vote but 5% of seats; etc. THAT intuitively doesn't seem fair or democratic. PR seeks to reflect % of votes with % of elected seats, which does seem intuitively fair and democratic.