Because 3,5" is obsolete for anything except spinning platters. More chips = more expensive and only enterprise wants to pay $ 1,000's for a single drive.
The problem isn't the horizontal space:
This was 2012, 4TB SSD 3,5":
This was 2014, 4TB SSD 2,5":
http://www.tweaktown.com/news/38249...ehemoth-optimus-max-for-inspection/index.html
This is 2015, 2TB SSD 2,5":
http://www.thessdreview.com/daily-n...pro-2tb-ssd-review-2tb-ssds-make-their-entry/
As the entire PCB of an 2,5" drive isn't even used, or the layout on the PCB efficiently for that matter, it's clearly not the issue (anymore) of not enough realestate. Stacking NAND is the way forward to reduce prices, power consumption and improve performance.
3,5" bricks are relics because of the spinning glass discs that HDD's had, and they were originally 5,25"
For SSDs, where they can make you a wedding ring out of storage if you wanted, there is no need to cling to an old format.
Enterprise has been adopting 2,5" (even for HDDs) too for a while for most storage needs, except for "cold storage" (slow, archive).
3D VNAND is pretty cool, but it does have one major drawback: a LOT of patterning stages. Even if you have a really mature process with a 99% no-defect rate, after 48 layers that's an effective rate of 61%! Even a 99.5% patterning success rate over 48 layers is a 78% overall rate. Of course, failures are due to more than just patterning (defects in raw materials, contamination during transport/handling, etc), but with that many patterning steps you're much more vulnerable to patterning errors like mask alignment, and to problems during the multiple chemical coatings washings between patterning, and unlike physical defects these are problems that often affect a whole wafer rather than one or two individual dies.
I was wondering if it indeed meant that stacking NAND also meant that defects per layer multiply the failure rate. That's why they want to keep the process "large", so they have more control on defects and allow a better failure rate, so more chips are good for use and the price goes down. Thanks for the info !