I think 1 factor is changes in the % of whisky being sold as Malt whisky is a lot higher now. In the past Distilleries most likely kept the best casks for Malt and sent the rest away for blended. Now they have more demand for Malt whisky so have to work with what stock they have. I know quite a few distilleries have had low aged stock lately.
If we're talking back before the -80's I don't think there were any thoughts about keeping the best casks for single malts, because the distilleries were not up to date on the single malt demand outside Scotland/UK. This was catered for at the time by the likes of Samaroli with an exeptional "nose" for quality casks. In the -70's he were allowed to rumage the warehouses of most distilleries and pick and choose casks freely, more or less proving that the distilleries didn't care what they sent of as long as they could sell by the cask.
I even think independent bottlers and blenders were allowed to pick out the best casks, and the distilley bottled what they could not sell. Which in it self is an explanation for the individual caracters that we today value as exeptional.
The blend market has always been and still is 1st priority for most of the larger distilleries, that's where the money is.
I had a tasting at Aberlour two years ago and we got a sample of this:
An amazing 21YO sherry cask, best Aberlour I've ever tasted.
All their stock is earmarked for the Chivas Royal Salute, and these bottles only exist as dealer samples of what goes into he blend
A few days later I visited Blair Athol, and on the first rack in the warehouse there were the last 3 casks of vintage 1959. The guide told us they had 7 casks the week before, but 4 had just been chosen for a very exclusive blend project. He wouldn't say what, but most likely a limited edition Johnnie Walker for the Asia market.