When it comes to motherboards, they have to develop the slot / physical form factor for the memory chips and get motherboard manufacturers to begin adopting it, whereas GPU manufacturers can just begin integrating the new memory on their boards when they feel the price/performance ratio is worth it.

Because it’s GDDR memory, not DDR. They’re two different standards that aren’t interchangeable.

https://www.technipages.com/whats-the-difference-between-ddr-and-gddr-memory/