Presumably because the cable would be (much) heavier and thicker. There is some movement towards having more pairs in newer cables, e.g. the Grace Hopper will

Presumably because the cable would be (much) heavier and thicker. There is some movement towards having more pairs in newer cables, e.g. the Grace Hopper will have 16 pairs, cf. https://cloud.google.com/blog/products/infrastructure/announcing-googles-grace-hopper-subsea-cable-system https://cloud.google.com/blog/products/infrastructure/announ...