The Guardian explains:
In the past, TV and radio programmes were broadcast from huge ground stations that transmitted signals at thousands of watts. These could be picked up relatively easily across the depths of space, astronomers calculated.
Now, most TV and radio programmes are transmitted from satellites that typically use only 75 watts and have aerials pointing toward Earth, rather than into space.
Thus, our TV signals—which could, if picked up by extraterrestrials, indicate the presence of an advanced culture (shut up) elsewhere in the galaxy—are now less detectable than in the past.
Additionally, the analog-to-digital switchover plays a factor. Think of it this way: the signal on my convertor box has been coming in pretty shitty since the switch to digital broadcasting. So imagine what it must be like for aliens who just want to watch NBC Thursdays (10,000 light-years from now, where they may not understand the concept of "Thursday", but still).
As we've been hoping that our signals are picked up in other worlds, we've been scanning the skies for inbound signals, too—but of course, the Guardian's Robin McKie adds, "What is true for humans would probably also be true for aliens, who may already have moved to much more efficient methods of TV and radio broadcasting."
In conclusion, it's not just Americans who don't watch the evening news anymore.