An 8x8 is way harder to produce. I mean, just look at the quality gap between 5x5 and 6x6 cubes. The 6x6 is just way harder to produce compared to the 5x5, and the same goes with the 8x8 compared to 7x7. And also 8x8 is not an official WCA event, which means it's way less relevant and people are not that interested, so there are not many 8x8s out there (remember the Dianshang one is the only magnetic 8x8 available currently).
