I think it's got to do with the independent bottling company. Here in WC we have Peninsular beverages that does the bottling for coke.
I also remember when Pepsi re-entered the SA market, they focused on joburg and released a 1.25l bottle at a lower price to coke's 1.5l bottle. Coke dropped their 1.5l in Gauteng and replaced it with 1.25l. We still have 1.5l coke here with a returnable bottle.