We can only guess.....
America's sports market is very saturated, tennis is pretty low on the ladder. When you have an American that's on tv all the time, etc., etc., your non-tennis fans will take notice. For actual tennis fans, I don't think it matters much.
We're a patriotic people and we like to support and pay attention to our own successes. If done to a reasonable extent, I don't see a problem with that. Moreover, we are used to having champions.
I mean, the way England latches onto its stars and even potential stars or the way the media pays such close attention to Lleyton and Flip's seemingly every move, it seems to me that even more attention is paid on those countries' tennis stars than here. So IS it really that much worse in the US? Maybe, I'm not sure.