Lets say you're a company working on developing, or are trying to sell some whiz-bang AJAX powered Web 2.0 mail client and you want to advertise, get the word out, grow your userbase. What's the best way to do that at the moment? Why, Google AdWords of course! They've taken over the world of web ads, supplanting a great number of the more annoying flashing image sorts. The best part is they're usually relevant to whatever site you're reading at the time! You do what you think is best and buy some ad space through Google. You think you've got the hottest thing going and want to make a serious push, so you pay a LOT for your ad placement. You want to be the #1 guy that is seen when any web mail ad is served up.
Well, since you're advertising a service in competition to what Google itself offers (gmail), chances are you'll never get that #1 spot, no matter how much you pay.
Check out this article from CentralDesktop as the author takes a look at what could easily be termed a Google monopoly in the contextual ad market.