The 30 year number was chosen because it is long enough to smooth out short term fluctuation from patterns (ENSO, volcanic eruptions, etc), but also short enough to capture real changes to an area's climate.
If you used the entire data set for normal, it's just going to be a trend line up towards warmer. If you use smaller chunks you can catch time periods that may have cooled or been wetter. It also is the WMO standard.