Domain Age - They've been around forever with tons of materials.
Quality - good enough for most things. The newer stuff is not that bad and usually more practical(read quicker to grok for someone in a hurry) that the more thorough MDN docs.
Dirty trick - association with w3 standards body (none in reality)
They used to be really bad. Their quality has significantly increased over the past years. I will use w3schools for quick info, but MDN when I really want in-depth information.