Why are the contacts for the Data lines in USB A and B plugs shorter than the contacts for Ground and Power?
Hot plugging. You want the power supply to be connected before the datalines. Some chips may latch if a signal is supplied to the signal pins before power is applied. This means the chip will internally short out and may get destroyed when subsequently power is applied.

In the old days you had to power off you computer before you could add a peripheral to it, that is not necessary with USB (and some other interfaces).