I'm not the commenter you're replying to, but I'll take a crack.
Saying it was a matter of "simplicity" glosses over too many of the details to be useful. Also consider that many things that early UNIX didn't include from multics got added back later (memory mapped files, dynamic linking, symlinks, ...) so by that measure it was maybe too simple.
A few points to consider:
* Multics was a joint project, and each participant had different levels of commitment and priorities for the project. This wasn't a recipe for success (see also: Taligent, OS/2)
* Hardware. UNIX moved to the PDP-11 soon after that machine became available. That machine went on to become the smash hit of the decade, especially in academia. The GE and Honeywell mainframes that Multics ran on continued their decades-long loss to IBM.
I've often wondered how different history would be if Bell Labs had bought any other minicomputer for the project. Probably today UNIX would only be known as a bunch of research papers. By skill or by luck they landed on the platform that would let their creation spread.
* Language choice. When the Multics project kicked off, using PL/I seemed like a progressive choice. After all, it had been designed by all the right committees! However, PL/I only had a moment in the sun.
To me, the single most impressive thing about the original UNIX team was that they also built C at the same time. This created a virtuous cycle where language-geeks wanted to use C (and thus wanted UNIX) and OS geeks wanted UNIX (and ended up learning C). If PL/I had lived up to its billing of being the true successor to Algol then C might not have had a void to fill and this cycle wouldn't be possible.
* Being born to the right (corporate) parent. Building even a small OS at that time cost quite a bit of money: not only to pay bright people for years to work on it, but to buy the minicomputers they needed. Bell labs had the resources and interest to do this but, uniquely, couldn't meaningfully productize it (due to their status as the telephone monopoly at the time) This meant that in the early days it was near-free, source code included. If UNIX had instead been developed inside of a computer company it would have been turned into an exclusive product and probably wouldn't have spread so far in its early days. Bell labs was one of the few organizations outside academia that would have done this.
* Just plain talent. The UNIX guys were, of course, flat out brilliant. Take pipes for example. Multics had the ability for processes to be chained, but it wasn't recognized as a first-class feature.. they were still thinking of programs as usually being separate beasts. The flash of insight was "this would be super-useful if all programs behaved this way and if the shell made it easy to construct" It's not a matter of "simplicity", but that they simply saw the problem in different way than everyone who came before. In short, UNIX was just damn good.
Certainly UNIX started out as a simple system, and that was part of its success. However, other things have been simple and never went anywhere. The point I'm trying to make is that to understand why UNIX won you need to consider a lot more of the history.
How do you figure?