I think you may be assuming what you set out to prove. X succeeded; therefore X never had any "serious competitors." I could equally well say that Linux succeeded because it never had any "serious competitors."
During Linux's formative years, the BSDs were hamstrung by a serious of lawsuits. Afterwards, they splintered into many different groups. Neither of these two things had anything to do with GPL vs. BSD. We could easily have had 5 small Linuses rather than one big one. The GPL doesn't prevent forks in the slightest.
The GPL is also less risky in the kernel. If your company writes some userspace code, there is zero chance that it will be considered a derived work of the kernel. (Linux even has a special preface to the GPL clarifying this.) With userspace code, things are a lot less clear. This is one of the reasons why Google banned GPL in userspace on Android. The Oracle vs. Google ruling that APIs are not copyrightable helps a little bit here; however, there are still a lot of ambiguities. What does the LGPL even mean when used in a programming language that doesn't have a linker? etc.
Personally, I mostly work on Apache 2.0 code at work these days. For my own projects I usually use Apache 2.0 or 2-clause BSD. The advantage of the latter is that it is usable in GPLed or other-licenced projects as well.