Actually, it very much was made with software project management in mind. The problem Trello was trying to solve was that the traditional way of doing tickets (Trac/FogBugz/Jira) doesn't give a manager a very good high level view of what everyone is working on at the moment and how projects are progressing. Trello was designed with that goal in mind and, frankly, does a pretty good job of it. It also works well for many other types of projects, because it is extremely flexible and unopinionated about how you do things, which was in mind as well, but was also somewhat incidental.

Source: I worked on Trello for six months before I left Fog Creek.