It is the nature of how affix tables are laid out. Suffixes on weapons, for example, are split among 2 tables, and it was largely chance how those probabilities ended up. For Aether, one of the suffix tables only has 1 rare suffix in it that is weighed towards Aether damage, but the other has 5. This largely came about from the affix tables being much smaller back in the day, where all of the affixes would not have fit into one.
Reworking the tables to smooth out the probabilities is not really worth it at this point.