While your technological reasoning is valid, it explains a factor of price difference but not a change of the difference which is the original question.

The "3D" process explains an additional factor coming into play at recent times. I bet there are a range of technologies which were developed for flash and used for flash manufacturing but not for DRAM manufacturing. This explains a growing disparity between the size/price ratio.

Let's not stop there. If those technologies are useful for flash, why are they not used for DRAM?

I see not hard reason why the 3D process cannot be made useable for DRAM.

The answer, I guess, is that the market forces lead to different priorities where the size/price ratio for DRAM is not so important and more development is done in other areas like speed, reliability, power consumption. So the real reason is not technological but market driven.