I believe GUI is a huge part of it. My rationale is that sweating the details of the GUI actually requires a profound amount of work, that is not at the top of the list of things that programmers enjoy working on. The the only way to get it done is to hire an army of well paid programmers to do it. And things like office and graphics software are nothing without GUI.

Second, some software builds on a body of proprietary domain knowledge that someone happens to own. I'd put mechanical CAD in that category. Probably the symbolic computing technology of Mathematica as well. In some cases, users of specialized engineering software actually put up with horrifying user interfaces because the software vendor has a lock on the domain knowledge.

The successes of open source include anything without a user interface, such as the infrastructure of the universe, and non domain specific programming tools.