We lack scaling from secondary stats. We end up being really really good at the start of an expansion, or a tier when Blizzard buffs our bases to the top, when everyone has little secondaries and relying primarily on Hit caps, primary values, and lower secondary stats, but we fall behind significantly as others begin to pull in tons of secondaries on the new tier's gear and we get very little benefit from them.
Go simulate our stat weights then compare them with a lot of other classes and specs. We get pretty good Agility scaling but the rest of our stats are basically worthless and only matter when compared against one another. We'll see 5.5 or 6 EP for Agility which is great compared to the 5 for other classes but then our secondaries aren't weighted any higher than 2.5 while other classes have secondaries valued in the 3s and 4s and secondaries improve at a faster rate than primaries. Us getting 1 more EP from our primary than other classes doesn't matter when we get 1 to 2 EP less on each secondary.
Mid tier as everyone has gotten the bulk of their upgrades, Hunters have been spiraling down.