I'm not sure. The pattern in the DRAM market seems to have been that only the biggest, most successful manufacturers can weather price crashes while still investing enough in R&D to stay on the leading edge. In the NAND flash market, we may simply not have yet seen any oversupply serious enough to do that, for all that we're in the middle of an oversupply-driven price crash.

It is probably the case that it's easier to stay in business with inferior NAND; Samsung beat everyone else to 3D NAND by a few years but Toshiba still made a killing off cheaper, lower quality planar NAND, and Intel/Micron didn't seem to suffer meaningfully from their first-generation 3D NAND being so slow. Now everyone other than SK Hynix has caught up to Samsung and the number of players in the market is actually increasing.

The recent scandals about DDR4 price fixing lead me to believe it's probably collusion.

Could it also be partly due to the infancy of the technology and the market? You don't see new RAM vendors and there are fewer avenues of improvement left to explore. That leaves a race to the bottom which is pretty unexciting for everyone.