Because its bad. If it was better than any other secondary even skill speed in terms of damage, that other secondary would be completely obsolete due to tenacity's additional mitigation effects. Nobody wants Tenacity for damage. And I personally have barely noticed any difference at all between 500 and 1500 tenacity. I personally wouldn't meld any of it because there is already so much of it on your gear that the materia slots you're wasting on it are gaining you probably less than 0.01% mitigation. Its just inefficient use of materia slots.
It just doesn't make sense to meld tenacity when our gear is already swimming in it due to diminishing returns, unless tenacity scales in some crazy special-snowflake way that we aren't privy to.