I always thought so too, but I think what's actually going on is that MDN is like an engineering professor, and w3schools is like a high school shop teacher. The former gives you information that is more correct and deeper but can linger too long on technical minutiae. The latter has a more superficial understanding of technicalities but a stronger grasp of practicalities and how to get things done. This characterization seems mostly fair from what I'm reading in this thread (beginners prefer w3schools, seasoned hackers prefer MDN), and it may well be that w3schools doesn't even need to do any advanced SEO hacking to overtake MDN because amateur web devs outnumber veteran professional web devs.