It doesn't really teach you anything in depth, opting instead to just spit out a bunch of examples.
As someone who used it initially years back starting out, I would have appreciated if it actually gave me context about how things worked and why rather than just copying and pasting code.
The only real reason it seems to get attention is due to the feedback loop of its ranking. Users see it as a top result and assume it must be some sort of authority on web development.
It has nothing to do with the W3C despite the name. Not to imply that beginners might know about W3C in the first place but still!
I wonder if it's still the same team of 3 developers running the place? Here's a writeup about it http://w3schools.sinsixx.com/about/about_refsnes.asp.htm
You spelt the very reason it is (was?) popular. The documentation/tutorial that gets traffic isn't the one with the most in-depth technical treatment, but, one which gives accessible examples to get started.