Plumbers on craigslist are, for the most part, doing lots of little 1-2 hour jobs. That means that there is likely lots of potential empty space in their calendar. They're probably lucky if they pull 20 jobs a week, so the rate has to go up to make a living.

IT work like you're talking about is probably going to be minimum of two weeks at 40-50 hours a week, so you probably have to pull 2-3 jobs a month. So the rate you have to charge to make a living, and the expectations of people hiring goes down as well.

Its the same effect as why I am paid $35/hr for an IT job (plus benefits, so lets say $45/hr) for a regular job, but wen we bring people in for contract work, we never pay less than $125/hr.