[Someone]

If, as Tanenbaum did, you look at it from a computer science perspective, Linux wasn't good, indeed. It was 'just' a Unix clone.

From a software engineering perspective, it left a lot to be desired, too. It basically only supported what was on Linus desk.

One could say the things Linux had going for it were a) a GPL license, b) being small, c) arriving at just the right time, and d) Linus willing to accept external input.

Whether a) is an advantage of course depends on personal preference. b) helped with d) because it made it feasible for many to download the thing; c) was luck; d) IMO was the most influential factor.