Take a look at http://quetzalcoatal.blogspot.com/2013/02/why-software-monoc.... The author points out how GCC's monopoly, though open source, forces clang to copy its implementation quirks almost exactly, because it's been the de-facto standard for years. Open-source doesn't automatically make the problem go away.
It was surmountable in gcc's case, but still took lots of resources (which clang has). It might not be surmountable in other cases, and even if it is it raises the bar quite a lot for new competitors.