I am not from the UK or Ireland so I might not be phrasing this correctly but thre are several rivalries (most prominently the Old Firm) that have distinctly sectarian themes. Though stuff like anti-Catholicism are a thing in the past in the US it was definitely a thing especially during times like the 1920s where Notre Dame was one of the most prominent college football teams. So why did no sectarian style rivalry develop from this period with Notre Dame or other teams associated with a particular identity (BYU, Celtics, etc.)?
Or am I wrong and sectarianism is a thing in US sports rivalries that I'm just unaware of.