1. Factory limits basically. There's a limit to the amount of fabrication lines that can create ram. Combined with the market incentives right now to make high bandwidth memory (HBM) over server memory (DRAM)... HBM starts as DRAM dies, so it competes with normal DRAM for wafer starts / cleanroom fab capacity.

2. Eventually more plants will come on line. Most of the main manufacturers have announced expansions but these can take O(years) to come online.