It's targeting a different market. The MDN web docs are excellent reference material, but if you're learning or early in your career they're impenetrable. W3Schools has a reputation of over-simplification (to the point of being wrong sometimes), but that's not at odds with being useful to people at a particular stage in their career.
For another example, I gather that most of the high school chemistry I learnt is wrong, but it was mostly pretty useful in understanding the physical world as far as I needed to progress in my education at that age. W3Schools can be useful in that respect.
