AT&T was not allowed to sell computer products (because it had the phone market monopoly), but US universities had heard about the work and wanted it. So the operating system's assembly code and source code was distributed for free to universities across the country.
The code was supplied "as-is", without support. (Programmers at universities found this an intriguing challenge, rather than a reason to fume.) The same AT&T group designed the C programming language in 1971, and in 1973 the Unix "kernel" was rewritten in C. Thus Unix could be ported to any machine with a C compiler - a huge portability advantage over other assembly- code operating systems. Universities could also "tune" the operating system (by writing extra bits of C to add to the kernel).
One of those was the University of California at Berkeley - which added some networking facilities to the original, making it useful for wide- area networks such as the Arpanet, father to the Net. But that was, at the time, small stuff.
Unix began to permeate more widely as university programmers grew up and looked for better-paid jobs where they could use their skills. But in the early Eighties, without a single corporate organisation pushing it, Unix was in danger of disappearing under the weight of IBM's mainframe operating systems. Its academic presence, however, saved it again. X-Windows was developed in the mid-Eighties at the Massachusetts Institute of Technology, and was quickly adopted by a consortium of Unix system vendors, who enhanced and hardened the system to commercial quality. It is now widely used on the powerful workstations that have grown to fill the middle market.
Then, came the Internet. Unix use has exploded, by stealth rather than marketing. When you hook up to the Net, you are almost certainly connecting to a computer running Unix somewhere along the line. It's an intriguing evolution of a product that has never had a marketing division pushing its development. One can only wonder what would have happened if AT&T had been allowed to sell it all those years ago.