It is my firm belief that Google works double time to make sure that email is a less and less viable marketing delivery format. It all became super obvious to me when Google tried to acquire Groupon. Groupon spurned a 6 Billion buyout offer from Google (bet they wish they could go back in time on that one). Shortly thereafter Google introduced the "Promotions" tab. In other words, if Google deems that the email is a marketing email they would "help" their gmail users by hiding the message in a special tab. This is all under the auspices of providing better service and making sure users see the messages they want to see.
Another example is how easily someone can mark a message as spam when trying to hit the delete button. Why is the spam button right beside one of the most commonly used buttons in gmail? I know I personally hit it by mistake all the time.
So why does Google work so hard to sabotage marketing email? Because they can't monetize it. Simple as that.