Because 250ml bottles would be bought by people travelling or for people's lunchboxes, rather than to have in the house. That's how I see 250ml bottles of any drink anyway.
I can see people buying the 1ltr as they do orange juice, in fact I can see 1ltr Serco cartons replacing the normal orange juice carton in the fridge. Every time they do the fortnightly supermarket run they will buy a dozen cartons. People will drink it for breakfast every day instead of normal juice. It will become a habit.
Why do you think 250ml bottles would be more popular?