Easy. They want to upsell you to the higher model!
Apple earns more if you buy the 17" model - so Apple's strategy in recent years has been to 'persuade' buyers to spend the maximum amount of money they can afford (even if they don't need all the features), by giving them incentives to buy the next model up.
The way to do this is to offer certain features only in the next higher model.
Perhaps Apple noticed in recent days that their 17" MBP orders dropped significantly as people went for the 15" plus the 8GB upgrade instead.
For many people the 8GB option is much more crucial than the bigger screen, matte option or better battery life simply because they intend to buy an external screen anyway at which point they get the matte option, higher resolution and don't really care about battery life as much as the MBP is being plugged in.
Without the 8GB option (if the firmware does not support 8GB even if the chipset would) quite a lot of people could be 'persuaded' to go for the 17" MBP even if they don't really need any of the other extra features.