Fashion shows are meant to provide an editorial direction for a given designer's line. Often that direction is not the most viable merchandising option for a given season. Thus buyers and merchandisers (internal or external to a fashion house or line) may only buy certain pieces or request that certain pieces be changed to make them more commercial.
According to Julie Fredrickson, a Fashion Marketing Director ,it is something of a myth that designers work in isolation as an individual artist. Fashion is actually very collaborative. Sometimes the designers revise their own lines depending on how they feel their vision changes as the season gets nearer. And realize it isn't just about a "lame retailer."
Every fashion brand or designer has their own internal merchandisers. Their job is to translate an editorial or design vision into something salable. I would say that merchants are just as critical to the design and editing process as a designer themselves. They are the great unsung heros of the fashion world.
Many designers can be unfocused and require the services of merchandisers to help them edit their lines into a more coherent vision.
So in the end it is a team of people that will judge what makes it to market. It is a combination of the designer, their merchants and often other parties like stylists and editors who will give feedback.