EVA is not so simple as jumping in a space suit and heading out the door.
Extra-Vehicular Activity (EVA) on the International Space Station (ISS) is a pretty tricky business. The suits astronauts wear these days take time and require help to put on. But here is a point that is not always considered: the atmosphere inside the space suit is not the same as the one on the ISS.
The air in the habitable compartments of the ISS is kept in a pretty comfortable state. The pressure is about the same as here on Earth at sea level. The gas content is also normal: a mix of Oxygen and Nitrogen (plus some Carbon Dioxide). The space suit is different, though. The pressure is lower, about 1/3 of an atmosphere, and it is pure Oxygen (along with any Carbon Dioxide and water vapor coming from the astronaut). The pressure has to be lower to allow the astronaut inside to move their arms and legs: if it were higher, the suit material would be too stiff to move. The astronaut survives the lower pressure because the actual Oxygen content is pretty much the same as that on Earth. So it isn't like they go up a mountain where the air pressure is low and the Oxygen content is low also. The pressure might be low, but the Oxygen is just as it should be.
Ask any diver what happens when a person quickly goes from high pressure to low pressure: the bends. Not something people would want to experience, especially not in space. There are plenty of other things to worry about up there. And lots of lovely sights to see. So astronauts need a way to gradually transition to the no-Nitrogen low-pressure conditions inside the space suit.
One way to manage the transition involves a process that takes two and a half to four hours out of the astronaut's morning. They breathe pure oxygen for a while before going out, with some vigorous exercise thrown in, to slowly pull the Nitrogen out of their bodies. It is difficult work on top of an already difficult day, and apparently not much fun.[1, 2, 3]
The alternative is to "campout" in the airlock overnight. The atmosphere is gradually adjusted while the astronauts sleep through the night. This way they are ready to head out about an hour sooner than they would otherwise. Imagine shaving an hour off your four hour commute. I'm sure you'd be thrilled too! 
Of course, this makes one wonder what would happen in an emergency (I blame that Gravity trailer). It seems that it would be impossible to rush the EVA procedure. The emergency repair that was done on the ISS just before Expedition 35 headed home was in planning for a few days at least. Chris Cassidy and Tom Marshburn went out to fix an ammonia coolant leak. I haven't been able to find anything on rapid emergency response for the ISS beyond hopping in a Soyuz capsule and punching out. That's a cramped, bumpy, but reliable ride back to Earth.
See also: NASA Spacesuit Engineer talks Space with Students on YouTube.