The previous Dragon cargo flight this year was a demonstration flight, this one is the first operational resupply mission.
Also relevant, the Cygnus spacecraft from Orbital Sciences Corp. is another ISS commercial resupply vehicle and is set to have its first demonstration mission soon (in early 2013). However, unlike the Dragon the Cygnus isn't designed around a manned capsule system and it burns up on reentry.
The Shuttle probably had similar launch windows for docking with the ISS. You need to match both the speed and position of the ISS to dock with it.
The ISS is moving at 7.7 km every second; if you miss by 10 seconds, you would need to (roughly) drop your orbit by 10km, wait 3.6 hours, then climb back up that 10km to make up for it.
Matching position is even harder. The ISS isn't equatorial. It swings north and south of the equator in a sinusoid.
It's probably possible to have a computer or pilot dynamically respond to all these things to make up for them. But easier to just deal with it by waiting a few days.
It was discussed during the webcast of the last launch, they said to save fuel. Presumably there's a more technical discussion around it, but that seems to be the gist of it.
With an arm capture they manoeuvre the craft a safe distance from the ISS then check it can hold position and move slowly toward/away. When they're satisfied then they edge it close enough to capture with the arm.
If it docks itself then you have to trust there won't be some weird last-minute f*ckup causing it to crash into the station.
Also relevant, the Cygnus spacecraft from Orbital Sciences Corp. is another ISS commercial resupply vehicle and is set to have its first demonstration mission soon (in early 2013). However, unlike the Dragon the Cygnus isn't designed around a manned capsule system and it burns up on reentry.