"Oh look I'm a giant warrior troll but for some reason I stop to cast in random directions during shadow dance, rotating my ass instantly up to 180 degrees while you lose globals autoattacking in your stupid puddle"
"Hi, I'm sub. I'm balanced lower than mutilate on Patchwerk, and I'm way fucking harder to run."
^This made me laugh pretty hard
I wouldn't say that sub isn't popular, it just isn't viable for progression. I'm sure if they made it more viable plenty of folks (including myself) would jump all over it. Looking at the next tier, SoO seems to be a tad bit more sub friendly than ToT but that positional requirement will still severly weaken anyone's desire to progress with the spec.
I love sub, I'm glad to see I am not the only one. I like to think I was decent at it. I played it in 4.2 until end of xpac and I used to rank consistently in top 50 on wowlogs. It's something, at least it means I was doing it right without any heroic gear. I miss it. I was hoping that with agility scaling and high Ilvl gear in 5.4 it will perform well again, but it still seems like 20-30k below Assassination on sim and that's with perfect conditions on top of that...
Just throwing this into the discussion - Cata rogues performance is extremely (and i mean REALLY extremely) biased by the fact the class went a bad balancing streak, starting very underpowered and Blizz buffed the class, not counting that we have always scaled a truckload with stats; this plus the legendary daggers made rogue way stringer than intended, partially also as a way to make the class more popular (and it clearly didn't work).
As for now, Sub is still a fun spec to play, the fact is that it isn't worth to play since it bring nothing more than the other two in any raid situation.