Not to be devils's advocate here, but I'd suspect Apple is aiming for a smaller retirement window for this kind of product.

It's basically a Laptop engineered in the iPhone/iPad space of the company, it's only natural for Apple to target a shorter lifecycle.

8GB RAM is maybe the best way to achieve that, many of the MacBook Neo buyers of today will be very compelled to upgrade to a newer (or higher-tier) model in ~3 years from now...

If the Neo would have 16GB of RAM today, it would be harder to justify an upgrade in 3 years from now, when the common entry-tier for laptops is likely still at 16GB...