I'm guessing power constraints. More flash you have, more power you'll require. Even increasing power consumption deltas minimally may impact battery longevity between charges on mobile devices enough to make it unfeasible with current battery tech.
I haven't thought of that, just another reason to get much better batteries. But to tell you the truth, I would trade very high definition 6" screen for more storage and a smaller screen.
"Power is an important consideration because the design of flash memories is closely tied to the power budget within which they are allowed to operate." http://cseweb.ucsd.edu/users/swanson/papers/TransOnCAD2013.p...