Sounds like you're not familiar with 'upselling'. Same reason jet black is only available with 128GB or 256GB.
They don't even write the amount of RAM on their official site so I don't think it is helping sales that much. I mean, the vast majority of their customer don't even know what RAM is, just us people on macrumors and other tech sites.
I don't chose the 4.7'' model to save $100, I just find it to be the right size for me and it really pisses me off when they reserve features to the Plus without a technical reason