Because we aren't fine the way we are?
We have three specs that play with extremely similar rotations, with very, very little individual spec identity (Assassin who kills people with melee weapons, assassin who kills people with poisoned melee weapons, assassin who kills people with melee weapons that make them bleed). We're one of the few classes that still operates on old, out of date mechanics (Energy and CP to use our primary stun which is already on a CD)--We've had all of our iconic abilities from over the years, stealth/vanish, sprint, stuns, shadowstep, combo points, auto-applied slows handed out to the other classes to make them more interesting, while we've gotten extremely little in return.We've lost more and more of our class identity as the years go by, poisons changed to act like shaman's magical weapon enhancements, stealth becoming less and less effective as the amount, size and variety of AoE in the game has increased and damage from stealth/uniqueness of stuns or silences has diminished.
Then there's our talent trees, which I think I can honestly say are probably the least interesting of all the classes. Most of our talents are either:
A) Completely required because they are clearly superior in the vast majority of situations (BoS, Anticipation)
B) Nearly useless because they are weak or one of the A talents is on that tier (Leeching poison, a talent that doesn't stack with our regular utility poison, forcing you to abandon utility for extremely low healing, Cloak and Dagger)
C) Hot fixes to class problems that rogues have been bringing up for years (Redirect)
D) Something that should be baseline (Night Stalker, Deadly Throw, Shadowstep, Leeching poison)
I believe we are also one of the classes with the most talents derived from talents we already had (i.e. we're one of the ones who got the fewest new talents) and I don't think it's merely opinion to say that most of the other classes got far more interesting talents both mechanically and in flavor. And the exact same is true of WoD, a passive energy increase, an awkward version of Dancing Rune Weapon, and DfA the one interesting choice. Really hard to be happy about those when other classes are getting things like Gladiator Stance, Ignite Weapons, Ravager, Permanent Doomguards/Infernals, Lava spewing and storm elemental Totems, Prismatic Crystal and the Meteor/Comet spells, Hurricane Strike, Direct Damage Shadow priests, Petless hunters and Exotic ammo (WHICH BY THE WAY IS BASICALLY JUST A MORE INTERESTING POISON MECHANIC), Death Knight Lich King talents.
We aren't fine the way we are.
We were fine the way we are, six to eight years ago, when all other classes were about the same, but we've been passed over every single expansion and major patch while other classes were updated and given new life; So that we are now left far, far behind the other classes in terms of interest, identity, and mechanics--And our steadily dropping population numbers show this quite well.