Why is my peripheral vision better for seeing in the dark?
Its down to the structure of your eyes. The eyes are made of rods and cones, which have difefrences in detecting light. 

Now, compare the images below.





As you can probably see, the peripherl vision is predominatlty due to your rods, and your central vision made of cones.


  Measured density curves for the rods and cones on the retina show an enormous density of cones in the fovea centralis. To them is attributed both color vision and the highest visual acuity. Visual examination of small detail involves focusing light from that detail onto the fovea centralis. On the other hand, the rods are absent from the fovea. At a few degrees away from it their density rises to a high value and spreads over a large area of the retina. These rods are responsible for night vision, our most sensitive motion detection, and our peripheral vision.


(Taken from: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html)

You can read about the photorecptors of the eyes here:
http://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/photoreceptors/