One possible reason is the greater availability of spell hit for other DPS caster classes. This has gradually negated the DPS advantage gained by affliction locks through Suppression. In other words, the spell hit granted by Suppression to affliction locks in early raid instances like Karazhan is really "ahead of its time", contributing to the build's stellar performance in those instances.
All three warlock talent trees get along well with our DPS role really, with different strengths in each spec. We are one of the lucky classes to get three different playstyles to do essentially the same job. The Demonology tree is well known for its survivability element - ever gone up against an SL/SL lock? Some demonologists can deliver serious hurt as well, as seen from some interesting statistics below.
Destruction banks on high shadow bolt burst damage and is often coined the "shadow mage" spec. Affliction is the king of sustained damage, with a full range of DoTs at an affliction lock's disposal. The interesting thing about these two trees is that their playstyles are probably as different as night and day. The destro lock is essentially a "one-trick pony", spending most of their time pushing a single button for shadow bolts. Since most of them will sacrifice their pet for buffs, pet management is also taken out of the equation. Affliction locks, on the other hand, generally have at least four, or even five spells in their rotation.
Less spells in the rotation means less micro-management. This gives destro locks more situational awareness in an encounter, which is essential for survival in raids where there is a lot of environmental damage. On the other hand, affliction locks have more to juggle, more decisions to make and are generally "busier" in a fight.
Many warlocks have leveled as affliction locks due to the excellent sustainability of the build. We are used to making snap decisions about DoTs on the fly and we enjoy the versatility warlocks have with respect to killing stuff: fear-kiting, drain-tanking and so on. Hence it is understandable if some affliction players complain that the destro playstyle is "boring" or a "no-brainer" - since the active interaction with what's happening on screen is cut down significantly.
Blizzard also seems to want most of us to become shadow bolt spammers in the end, as Warlock tier 6 gear favors shadow bolting locks. "Why is Blizzard not making gear tailored to our favorite build?" is the question asked by many affliction warlocks.
It is however interesting to note that on the WWS Scoreboard, a neat ranking chart based on raid data pulled from WoW Web Stats, just 10 of 15 top Warlock DPSers are 0/21/40. Out of the five who aren't destro locks, two are demonology, one is affliction and two have hybrid demo/destro builds - building for Soul Link probably.
These numbers suggest that while 0/21/40 is the favored build, it doesn't seem to the runaway cookie-cutter build yet. Just as we have various outfits for various situations, there might be reasons to dip into supposedly less DPS-oriented trees to gain certain key talents, particularly for greater survivability during some encounters.
In light of this, raid leaders should keep an open mind about warlock builds and not insist on the "standard" build for warlocks under their charge. On the other hand, affliction locks who are fretting over the simplicity of the destro build should also look for a build and playstyle that really works for both themselves and their guilds. Yes, 0/21/40 is a popular endgame build, but it is not the only way to play your Warlock.
In terms of gear, while other classes may have multiple sets of gear meant for different roles within a tier, like shamans or druids - I don't think that our trees are diverse enough to warrant three different sets of tier 5 or tier 6. In the words of a wise DPSer, "Less QQ and more pew pew!"
V'Ming still spends his time laughing ominously, but in the hallways of Tempest Keep and depths of Serpentshrine Caverns now. He is also looking forward to tanking Leotheras the Blind in the near future.