Why don’t they order more if it goes out this fast?

Maybe they don’t order more 'cause there’s not enough storage space or the contract runs for a very long time. Keep in mind that drink producers probably want guarantees as far as their sales are concerned, so they want contracts that run for a long time. It wouldn’t make sense for manufacturers to invest into an expensive machine when they can’t be sure to sell all of their products for many years.

So now this shop you go to might want to get the cheapest contract they can get to maximize profits and those are usually the longest running contracts. If during this long time sales for this drink ever drop then they would end up with a lot of drinks that they can’t sell, which requires even more storage space or selling the products cheaper or worse yet disposal. That risk could be a reason they don’t want to order more.