Customers were complaining to DEC, SUN, IBM, etc about how confused they were using UNIX (think of the same Linux fragmentation today.) Major customer groups got together and said they would switch to Windows if the companies didn't create a stable environment for UNIX work. So the company CEOs got together before the "official" POSIX meetings to decide how little they could standardize (and cut their own costs) while still allowing them to have their important differentiators for sales. They decided that integrating the programming API and some minimal administration aspects would be the right approach. So they then got staff involved in the series of POSIX meetings to decide the spec (not telling them explicitly about the initial agreements between the CEOs)
Really, there is a lesson here. Customers (us) need to force the Linux vendors to create a more uniform experience for us. It doesn't "just happen". There is too much arbitrary variation (with no real benefit) and the learning curve is just absurdly hard for working with Linux (which is old enough that all the basic operations should be quite stable - and mostly quite simple.)