Solid-state storage devices and the block layer

1. Addressing: DRAM is byte/word addressable; NAND flash is not. NAND flash pages are currently 4KB in size and must be read/written
2. Flash management: flash sucks. It has long erase times, needs wear-leveling, needs lots of ECC and redundancy to be reliable. Dealing with flash requires a lot of careful management that nobody's going to want on a DRAM-like path.
3. Speed: flash is a lot faster than disk. But it's still a lot slower than DRAM (a write to a busy NAND part may have to wait up to 1ms).
4. Size: it's very expensive to try to address a terabyte of DRAM. 64-bit CPUs don't actually implement a full 64-bit address space. It's much cheaper to just address huge storage devices in blocks, like a disk.

If in a few more years phase-change memory becomes big and cheap enough to give NAND flash a run for its money, then it may be time to start treating nonvolatile memory sort of like DRAM. But that day isn't quite here yet.