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Not sure why exactly
Anecdotal but it could have had something to do with the piss poor upkeep brought on by trade in options and maybe even extended warranties. I spent a lot of my youth being dragged around service shops that one by one folded through the 90s. A reoccurring comment when i asked them what was happening was that no one could or could be bothered to maintain there car anymore.
I suspect if you asked them today they'd say it was because of the forrins and tofu mob.
Second hand cars in the UK were abnormally cheap compared to the rest of Europe for a long time. Not sure why exactly, but the increase in price just takes it more in line with the averages elsewhere.