Because most of the world of warcraft players including the ones reading this arent that good at the game including the tanks.
A lot of tanks arent bad players some are but they arnt the ones people complain about being elitists.
Most DPS are there for a free ride and arent paying very much attention because there are 3 of them so they are ether spamming buttons while talking to a friend or the exact opposite they are watching YouTube clips while alt tabing back and forth.
The button spammers just pick the closest target to them and start DPSing it even if the tank hasnt even touched it they dont care they are topping the meters while half afk and bragging about it to their friends how bad the DPS is in the group. Then when they have aggro they blame the tank. Alot of these players are rogues and hunters.(obvious reasons why they are retarded)
The alt tabbed half ones watching YouTube clips (or any other thing they are doing) are in full ICC gear and are doing nothing to help the group they have 5k+ dps and only 5% of the total damage done.
Then there are the people that just hit 80 and have no idea how their class works yet they think they are doing very well because they only saw 20 misses instead of 40 they are the ones that have berserking on their weapons as hunters or int/mp5 gems as warlocks.
Next we have this issue: The DPS forgot that they can do stuff besides DPS.
The tank is pulling all the mobs taunting generating threat on 3 differnt mobs that all 3 DPS are hitting. Then the DPS pull an extra group with casters for w/e reason and the tank has to run over and pick up those mobs interupt the casters and then pick get all of the mobs together so the DPS arnt complaining they cant AOE all of the mobs while there are casters in both groups. Mean while all they had to do was Mis-Direct TotT CC or interupt to make the tanks job a little easier but dont.
So in most groups you have a tank that is working his ass off and 3 DPS that are doing one of the things listed above. The tank is #1 or #2 on recount for damage done has all of the interupts and has all of his ablitys on cooldown almost all the time.
Meanwhile the tank is wondering WTF are these dudes doing. Im in full tanking gear and #1 DPS? Why is this DK death griping the add thats on me. Why did the hunter send his pet into that group over there? Why did the healer put a shield on me as I charge into the group of mobs now I have to go pick them up instead of just having them come to me?
Next is the healer that doesnt drink or use mana return cooldowns between pulls. He waits till hes completly oom then spams OOM 10 times in chat while you are tanking one mob waiting on him to drink. However instead of finishing his drink he jumps up and spam heals you while you are at full health. Then proceeds to act as if you cant see his mana bar and tells you as if this is the first instance you have ever tanked how mana and healing works.
The last one but is far less common than the rest:
The tank thinks hes good.
Normaly this tank is from a bad guild that has a few good players in it but hes their main tank he gets fed tricks and misdirects from his DPS so they dont pull off of him and he doesnt know or care because he thinks hes doing massive threat so when he trys to do heroics he doesnt have these things being fed to him and people pull off him. So he blames them.
These are the things I've ran into in the 4+ years Ive been tanking on a warrior. I have probably missed a few but these are the major ones that are far to common in heroics. People wonder why tanks get bitter or act like "elitists" this is why I hope you understand it and stop posting about it.
EDIT: I shouldnt have to put this but this post doesnt say ALL dps are bad its refering to DPS that act in this way get over it if you are offended because a tank that doesnt know you treats you as the large number of people he runs into and Im sure hes thankful he ran into the 1 in 50 (made up number [for the trolls]) DPS that actually does well.
Edit 2: I cant believe the number of people that are offend by this post YOU SAID SOMETHING ABOUT A HEALER (OR A DPS) AND I'M A HEALER (OR DPS) THIS COMPLETELY OFFENDS ME EVEN THOUGH YOU WEREN'T DIRECTING THIS AT ME BECAUSE I DO MY JOB I'M NOT PART OF THAT GROUP THAT YOU ARE TALKING ABOUT!! FLAME FLAME FLAME (um great dude you don't do the things listed.. um... GOOD JOB BUDDY KEEP IT UP!) If you want to defend piss poor players that do the things above Ill have a great time making fun of you. If you are that stupid you deserve it IMHO.