For as long as there's been national news, it's been dominated by a few headline sources. Before that there were lots of local sources, but nobody was reading the local news from both Connecticut and Colorado.
I don't know what the difference is today. Maybe there isn't a difference, we just prioritise modern problems over the same problems in ages past. But I really don't think the answer is a narrowing of sources, unless your point of comparison is something like fifteen years ago, for the small subset of the population who were mostly getting their news online at that point.