Was Roddick really the "hero of American tennis"? I wouldn't consider him that - he wasn't a consistent or popular enough player. IMO, there hasn't been one of those in a long while. Maybe since Andre retired.
I see what you're saying though and if that's part of it, then Isner needs to wake up. There are PLENTY of American men over the years who didn't become the big star they maybe could have of should have been. To be the big man in tennis, you need more than just ability - and he really hasn't done that much in career yet. Most people know him as the guy who played that REALLY long match at Wimbledon and it's not like that match was against a major player either. Heck, he could pay his dues, win some slams and still not be "the guy." Look at Jim Courier or Michael Chang. They were always overshadowed by Sampras and Agassi.
Speaking of Jim Courier...I really would prefer Mary Carillo call the men's matches on CBS as well instead of Courier. She is SO much better. I hate that little subtle sexism that seems to say that women should call women's matches, but not men's.