A lot of the early open source used to recreate the SunOS commands and args. It's what people were used to. Docs were also accessible in a way that POSIX wouldn't be for a decade .. after that, it grew from there and SunOS started to feel dated.
It was even more than that. SunOS has some extensions (thread in particular) that pre-dated pthreads. A number of us had built pthreads packages because of Posix, but found ourselves building threading packages in the key of SunOS and later Solaris - why because the ISV's were using Sun's threading scheme.
The "why" is simple --- ISV capture for your target was (and still is) the most important driver of selling new platforms. The better job you do in making it easy for someone that has an application, the more attractive your target becomes.
Ted sometimes has mentioned the other "Golden rule" about "he who has the gold." As the creator/supplier, it is hard to be magnanimous when you are ahead and it is often difficult to acknowledge real reason >>why<< you are ahead. Plus the people on the other side of the tech delivery (the users), are more driven by basic economics - what is the most cost-effective way to get your job done [ i.e. this is a classic Christensen disruption].
IBM lost the Research/Universities to DEC which started out being very open and easy to work with and extremely cost-effective. As more $s piled in the market, DEC started to be more and more protective (and moved more and more upscale). To many at the time, DEC compared to IBM (Mainframe S/360 vs. PDP-6/9/10) again -- worse technology, but 'good enough' (and a new growing customer base). The Unix Workstations come out - again 68K vs. Vax (story repeats). Sun eventually taking the lead from DEC. As Larry points out, Sun certainly started being extremely friendly to the same group -- again cost-effective and leading tech. Sun went upscale and the Intel/Microsoft alliance was good enough to a lot of people.
I supposed there is the pride of owner/developer-ship; but to me, but the whole Linux vs. BSD (or SunOS or MacOS for that matter) is a silly argument (and wish people would get over it/themselves). Linux (particularly on INTEL*64) is the current (popular) and cost-effective implementation of Ken, Dennis, Doug, et al. ideas. No more, no less. Thank goodness it is cost-effective and accessible to all of us and available for us to use to do what we need and want to do. But let's stand on each other's shoulders, not step on toes for some injustice (believed or truly real). A lot of us and in a number of different places go us to where we are today.
For us UNIX historians, we need to be careful and learn from our own history here -- the Cell Phone/Mobile target is the engine for the next Christenian style disruption. It is by far the #1 target for people writing new programs (which I find a little sad personally - but I understand and accept -- time has marched on). In the end, a small mobile target will be the tech on top, and available will be driven by market behavior and those suppliers will be "who has the gold."