Hmm... It might be like asking why Justin Bieber is so good, that millions world-wide are going crazy over him, while some really good local bands are largely unknown outside of your city/country. It is hard to make something for everyone, so games like WoW and CoD have to compromise, in order to satisfy an average player.
Another example from my personal experience - there are around 10 kebab/pizza places, in a 5 minute walking distance from my home. I know what place serves best pizza, best kebab, best burger, etc., but someone who never been to Valby, might go for Burger King or Dominos. Not because they like Burgers from BK above any other burger, but because its easy, cheap and you know 100% what you are getting. What i mean, is when someone has only 2-3 hours a week to play video games, he will not bother with Mount and Blade, he will go after a well known, well marketed product because it is convenient. And big companies give good polishing to their games, so even if the gameplay is nothing special, it will still satisfy your basic "gaming needs" without having to endure a year long patching, and all the "raw edges" that will never be fixed in a low budget game.
And some games are just great from the day 1 - think about the first Left For Dead
Huge success because it was both polished,
original and correctly presented to the public.
Since I wrote all this, i will also use an opportunity to "promote" this great band:
*had to fix the link, as it was cutting out on 1:04 :(