[OnyxRider]

Well I can tell you a few reasons for unfinished builds from some "builds" that I have aquired from a collection. Most of these are from the 50's and 60's and collected by a man who specialized in collecting low production american muscle cars. He also spent some time hunting down builds that he heard about for years. He got a rumor of a muscle car or unfinished build and he would find it or found out it never existed.

About 10 builds are still warehoused and I really had no interest because these came with the American muscle collection I purchased. The owner's wife eventually passed and he became a caretaker for me.

Only one I had completed a Byers SR-100. The others are not really big money cars and someone down the road may do something with them.
There is a Kellison that I would like to finish some day though.

Here is an answer to your post, sorry always a story behind a story behind that one.

One build a young man started while in high school and attended college...never had the time to finish. Another was suppose to be a family build and never happened. One a "Shark" was handed down several times and I guess all the build info was lost. This was decades before the internet and it sits.

The rest no one knows why they were left unfinished but the reasons listed by others sound reasonable.