Commercial UNIX was built to run on expensive RISC hardware; Linux was built to run on cheap x86 hardware. Commercial UNIX was certified to run Big Corporate a

Commercial UNIX was built to run on expensive RISC hardware; Linux was built to run on cheap x86 hardware. Commercial UNIX was certified to run Big Corporate applications; Linux ran FLOSS LAMP stacks. When x86 hardware became cheap AND reliable, and web apps took off, Linux became the better choice. The commercial UNIX vendors didn't adjust fast enough and were left behind. And now we have a network effect, where very few developers/ engineers know HP-UX or AIX, and software is being written specifically for Linux that would have to be ported to HP-UX or AIX.