I guess, this i common because allmost all encounter are casterfriendly, and stacking (SHADOW) priest is wonderful for substained dps/hps of all of your manabar user. You will not find many successfull raiding guilds with stacked melee through all the instances. But there are a lot of guilds, using 2-3 hunterspots as intended (assuming you spend 2.78 slots per class on average).
And you shouldn't forget, hunter cycles aren't that easy to master then hitting your hotkey, waiting for next energy tick or global cooldown. That's why, there are less good hunter then good other dps out there. Nobody need Mr #10-15 in your overall damage a second or a third time, one misrediction should do it.
You posted some wws parses in the other thread and have proven, that your damage is quite fine. Why they should switch you?
I can't look at the thread (proxy..), but how are those numbers collected? Did a bored human beeing arsenal'ed through all known raiding guilds, that slay their way through ssc/te+? But how to check the attendance and regular composition etc.? What about 7 shadow priests, all with different working schedule, only can participate in 20% of the raids?
I don't give much on those statistics, there are to many factors, a foreign can't look at.