The RtI campaign is more of a tutorial than a real campaign. It's based on an interesting and quite important event, which is appreciated and known not only by military history freaks but also by normal people.

Most people don't give a damn or even know the 7 years war, the Northern war and the Austrian succession. Everybody have heard of the american revolution. Furthermore, it is a good way to cater to north americans, since their continent had more or less been left out of previous TW games.

ETW is not American Revolution Total War. Completing the RtI campaign doesn't take more than a few hours, and you don't even have to do it: you can start a Grand Campaign right away .