Maybe because of the adoption of plastic bottles, which cannot be sterilised and re-used? Ask Lizzie.....
Ron.
Because the fizzy drink producers realised that it was cheaper, more convenient, and better for their business to swap to plastic, which could hold more liquid (now up 3 litres) and therefore could increase the amount being sold.
In addition, the storage of empty bottles and crates used up valuable space in retail units, which as supermarkets grew bigger and space was, and is now continually measured for it's retail sales value, the cost of storage was unacceptable. More importantly, the transport used, and handling methods changed, with instead of the 1960's drinks lorries designed for small deliveries and collections of empties, large covered, open sided trucks took their place. A single driver, with fork lift assistance from the warehouse staff of CDC's, could unload tons of bulk drinks quickly, and not be held up with loading empties. The reduction in costs was dramatic, efficiency was much improved, and more pop was sold in bulk than ever before, unloaded from larger and larger lorries that were cheaper to run and could carry tons more of product on pallets.
Now the planet is paying for that "advance", so now the public will have to swallow the extra financial costs incurred by winding back on the "progress" that reverting to smaller, heavier glass bottles, will bring. But, if we can save the planet, I am all for it!