Because reading from an RSS source required you to have an RSS reader of sorts and there was some configuration required on the users' part - Install a client/setup an online tool, this was an extra (rather complicated) step as compared to say, simply going to Facebook and subscribing to a page you like with the click of a button.
Now, I'm not discounting the possibility it may have been killed strategically by the internet giants, because it's not in their interest, but I do believe Google Adsense (at that time) did allow you to publish ads into your RSS feeds somehow. So, maybe they killed it because of poor adoption rates and the configuration required to setup one?