My bet is they asked those lovely companies that methodically strip the taxpayer of billions a year - you know the ones I mean - while giving very little in return for a quote for microfilming/microfiching the slips, got quotes for several million so decided it would be cheaper to just shred them.
One of the companies that strip etc probably received only marginally less for shredding them than they would have got for filming them but, hey ho, look at the money we saved.
For any buyer other than a government department the cost of filming the slips would have been trivial, a few thousand or thereabouts. My theory as to why such a relatively simple administrative task wasn't done is all too likely. Sadly.