Why is the common lead of unipolar motor connected to the high level voltage?
Silicon switching devices intended for low side switching (ie, the ground side) such as N-FETs and NPN transistors utilize bucket brigade displacement of free electrons in the silicon as charge carriers.  Conversely, the simple form of high side switches use positive charge carriers - the so called "electron holes" which are less mobile than actual electrons.

N channel devices work so much better as switches, that in bipolar systems which need to switch both sides, above moderate power levels, it's often chosen to build more complicated level translation circuitry to boost the control signal and allow using N channel switches on the high side as well, rather than lower performing P channel devices.

But for a simple unipolar setup, you only need to switch one side, so you chose to switch the low side where high-performing N channel devices can be used directly.