The problem has been that historically W3Schools has been inaccurate. MDN set out to fix that and be the best source. I don't know why W3Schools has so much SEO juice.
Historically maybe but I don't think that's true nowadays.
Even then, why would you support a closed, commercial, outdated website instead of MDN which is essentially a wiki?
The same reason why beginners get a sentiment of attachment with websites like Stack Overflow.
They're looking for "how to do <something>", not for the docs.
I find both to be very well done but MDN goes in depth the most.