The reasons for this are many and complicated. One partial explanation — or result, depending on how you look at it: Appeals to patriotism work better on older, whiter Americans, nostalgic for a national unity that looms larger in gauzy memory than in fact (something that has not gone unnoticed by marketers). Trump’s fan service to “my people” only highlights and amplifies the trend.
Like appeals to divine authority, appeals to patriotism only work on people who recognize the authority of patriotism. And the more you invoke patriotism as a substitute for fact-based arguments, the more you drain the power from patriotism. The more patriotism is used to sell an explicitly partisan agenda, the more patriotism is seen as a partisan phenomenon.
But there’s also the broader philosophical problem with nationalism as a political program. If your defining concept of politics is “national unity,” it is almost impossible not to succumb to the statist temptation over time, because the national government is the only institution that claims to speak for all of the people. But by definition, there are very few things in a democracy that enjoy anything like national consensus, which means the party out of power will feel steamrolled and lied to (see: Obamacare). And from a conservative perspective, some nationalistic things — like, say, nationalizing or socializing industries (which are the same thing) — shouldn’t be done even if there is a national consensus. The same goes for patriotism. Nationalists or populists might want to round up, say, Japanese Americans and put them in internment camps, but I like to think patriots would have objections.