The more likely reason is that it was a test machine, as shown by the paperwork in the cabinet and the low play count. It was put on location for a few weeks to gather earnings data and then probably pulled back to the factory. A lot of those test/proto machines were then scrapped or sold at a very low price to someone on the design/production team if they had the means to take it home.
Back in this era it was very rare for an arcade cabinet to be sold directly to the home. Aside from being very expensive, you had to buy from a distributor much like a car dealer. Most local distributors hated dealing with home owners (too many questions, couldn't fix it themselves, delivery was a bitch, etc).
Given this was found in a Chicago suburb I'm going to take a wild guess and say it was a Bally/Midway employee that kept it in their garage or basement for a few decades and then decided it just took up too much room. Or it was handed off to a friend or neighbor over the years but either way it really hasn't left the city and never saw hard use in an arcade. Galloping Ghost has taken great advantage of this situation and obtained many pieces that are more rare then Discs of Tron.
The reason I know this is because I have a few engineering sample games in my basement as well.