The answer is burried, here is the only real reason given: When MacAskill replied: “The main reason for only a small percentage [being published] was diminishing interest [from the public]...” This is insane. They have 99% of the leak under lock and key and they say the reason is because no one wants to see it? My guess is its a gag order/request, if they publish they will lose other access or something but why would…
Remember, the purpose of journalism is to make money - news is a product like any other. A story people aren't as interested in anymore is a story that doesn't make as much revenue.
To make money is the purpose of journalism as much as eating or breathing are the purpose of your life, i.e. they are not.
They're just the bare necessity that needs to be done to continue to exist so that you can do more 'important' things (see Maslow hierarchy).