This question is really only answerable by Game Freak. However, we can theorize.
Regieleki is essentially the embodiment of electricity. Other Electric types that harness electricity (Raichu, Manectric, Electrode, Jolteon, Zebstrika, etc.) often have very good base Speed stats. Electricity flows quickly, faster than many other things. It can travel close to the speed of light (about 186,000 miles a second), which explains why many Electric types are so fast. Bulbapedia claims Regieleki is based off a light bulb, an electric charge, and an electric generator (source). Anything that can produce its own electricity essentially gives it never-ending power, meaning it can travel at almost the speed of light if it so desires, which explains why it's so tremendously fast.
Hope I helped!