That is a good question. I'll try to answer it from the perspective of being in IT for about 4 years now.
There are 3 departments in every company that hardly anyone likes:
1. HR
2. Accounting
3. IT
The primary reason is because these departments essentially restrict what people can do (can't harass anyone, can't spend money on everything, can't install your games). This situation already breeds bad attitudes.
On top of that, the only time anyone talks to IT is when something is wrong/broken. How would it make you feel in your job field if you only talked to the people who had problems, and were more than likely unhappy because they have a problem? I've never EVER gotten a call/e-mail from someone saying, "I'm just calling you to say what a good job you are doing." I might get a similar comment after I fix their problem, but never before.
About the condescending attitude. For a second, I'd like to compare IT with automobile mechanics. People use computers like they use cars: They use them everyday and because they are complex systems, they have no clue on how they work. A majority of people know when they get in a car and turn their key, it comes on, and they can drive. That's about the extent of their knowledge. When a user turns a computer on, they expect to be able to do whatever they do without any problems (check e-mail, surf the web, etc).
Along that same comparison, lets say something breaks on the car. You go to the auto mechanic, he fixes your problem and he tells you that you actually have to change the oil in your car every 5,000 as preventative maintenance. Of course, the auto mechanic will be a little condescending because he believes that to be common knowledge. Everyone is supposed to know they have to change the oil in their car. Everyone is already supposed to know they can't drive their car like they stole it and expect it to last forever.
The same can be said about IT. When a user brings you their computer to fix it, you of course see they have 40 internet explorer toolbars installed, 1000 viruses, and a million shortcuts on their desktop for free ipods. You as IT believe it to be common knowledge that a user can't expect his computer to operate efficiently if they "drive it like they stole it."
In both cases, people just don't know. People assume they know, but when you tell them they are wrong, they immediately go on the defensive and don't want to hear anything you have to say. That generally happens any time you tell people they are wrong. Now you have an IT professional working with a pissed off customer. Multiply that a few hundred times, and the IT professional will automatically assume the worst when working with a customer, thinking they are all the same (stupid, ignorant, etc.). I bet it is quite similar to how auto mechanics feel, or any other person who repairs complex systems that people use and take for granted everyday.
Anyway, enough rambling on. There is my $.02 about the topic.