DRAM hit a scaling wall about a decade ago. Capacity per dollar has not been improving significantly, certainly not at the exponential rate it had been for the prior 50 years or so.
As to why, search for words like "dram scaling challenge". I'm no expert but I believe capacitor (lack of) scaling is the biggest problem. While transistors and interconnections continued shrinking, capacitors suitable for use in DRAM circuits ran out of steam around 2xnm.
https://thememoryguy.com/dram-prices-hit-historic-low/
As to why, search for words like "dram scaling challenge". I'm no expert but I believe capacitor (lack of) scaling is the biggest problem. While transistors and interconnections continued shrinking, capacitors suitable for use in DRAM circuits ran out of steam around 2xnm.