The first problem is that presidents are not historians or theologians, and in political rhetoric it’s hard to escape from oversimplication. You can introduce the Crusades to complicate a lazy “Islam violent, Christianity peaceful” binary, but then a lot of Christians are going to hear an implied equivalence between the Islamic State’s reign of terror and the incredibly complicated multicentury story of medieval Christendom’s conflict with Islam … and so all you’ve really done is put a pointless fight about Christian history on the table. To be persuasive, a reckoning with history’s complexities has to actually reckon with them, and a tossed-off Godfrey of Bouillon reference just pits a new straw man against the one you think you’re knocking down.
The second problem is that self-criticism doesn’t necessarily serve the cause of foreign policy outreach quite as well as Obama once seemed to believe it would. Early in his administration, especially around his 2009 speech in Cairo, there was a sense that showing Muslims that an American president understood their grievances would help expand our country’s options in the Middle East. But no obvious foreign policy benefit emerged, and since then Obama’s displays of public angst over, say, drone strikes have mostly seemed like an exercise in self-justification, intended for an audience of one. (Meanwhile, our actual enemies can pocket his rhetorical concessions: The alleged relevance of the Crusades to modern politics, for instance, has long been one of Al Qaeda’s favorite tropes.)
A third problem is that Obama is not just a Niebuhrian; he’s also a partisan and a progressive, which means that he too invests causes with sanctity, talks about history having “sides,” and (like any politician) regards his opponents as much more imperfect and fallen than his own ideological camp. This can leave the impression that his public wrestling with history’s tragic side is somewhat cynical, mostly highlighting crimes that he doesn’t feel particularly implicated in (how much theological guilt does our liberal Protestant president really feel about the Inquisition?) and the sins of groups he disagrees with anyway (Republican Cold Warriors, the religious right, white conservative Southerners).