Because SP2 deleted a bunch of SP1 files with the new Clean-up tool. Microsoft hit a home run with this service pack.

“SP2 also includes a Service Pack Clean-up tool (Compcln.exe), which helps restore the hard disk space by permanently deleting the previous versions of the files (RTM and SP1) that are being serviced by SP2. The Pack Clean up tool can also be run offline while creating slipstream images to reduce the size of the image,” Microsoft informed.