Chose my 1st army (Empire) because the detachment system looked cool. Plus, I most often pick humans when playing PnP
rpgs or video games.
Picked my 2nd army (HE) because it was
fully painted 
and the price was right.
My (still on the sprue) 3rd army (
VC) was chosen in concert with my decision to make my 1st army from Stirland. A Stirland force that could afford a Steam Tank and Greatswords had to be getting funding from a
shady mysterious source. Also, I can start them off slow as a SoM sorcerous pact and eventually build them into a full army.