Well, not all secondary stats are created equal... Who cares about a (rather high-end) health +5% or tenacity +5% ?
It's more like the scale of the bonuses (and their nature) varies widely: health, defense, tenacity bonuses are way too low to matter anyhow in a battle, whereas potency, protection and crit damage are pretty decent.
Speed is a bit of a special case, as mostly what people want is not "more speed", but "go first". If those 2 things were 2 distinct attributes (say there's a mod that adds +%TM at the start of a battle, and only then), which one do you think people would rush to?
