I wish I could find out specifically what went wrong with Visual Thought.
Visual Thought was not an OS or some immensely complex distributed workflow product. It was a simple diagram editor. It just did its job very well (probably because it didn't try to be too much). It just couldn't have cost all that much to keep it up to date with the different OS's.
Not sure how many people have used it, but it was head and shoulders above anything else on Unix at the time; and I still can't find anything on Mac in 2017 as good (it was much better than Visio at what it did as well).
And it was popular. They must have had decent revenue.