Existing FPGAs have limits on amount of memory supported. There will generally be many more FPGAs able to support 1 GB than ones able to support 8 GB.
ASICs, on second thought, should not be a problem as long as the DRAM required doesn’t fit on chip, which is certainly the case at >= 1GB.
There is another potentially large benefit to requiring more than 6GB. Many huge mining farms are dominated by <= 6GB GPUs. Excluding them makes it easier for hobby miners to compete.
Lowering memory requirements allows more people to mine with their current equipment, but most won’t be able to do so profitably.
That said, we could in principle support a range of acceptable cuckaroo sizes rather than the single 2^29 size, if deemed beneficial to adoption.