Edit: Nevermind apparently this is incorrect. This is just my take from trying to understand why we were Catholic and not Christian, and then avoiding being mistaken for 'Christian' later in life and not clarifying and staying in my lane. But I guess there is something else I don't understand going on.

'Christians' in the US don't consider Catholics 'Christians' (using quotes to show it's a group identifier based not solely on following Christs teachings). They (Christians) are also the newer splinter group. So Catholics have taken to identifying themselves as Catholics to 'stay in their lane' or to self identify to the 'Christians' that they aren't part of the accepted/in group. Catholics were lower status historically, look at the talk around Kennedy. Also Christians might not want to be friendly with Catholics but they would with Christians, so you out yourself as Catholic from the start to avoid that bullshit (so back to 'stay in your lane').

The two have very different approaches to religion, with Catholics following the 'love they neighbor' 'care for the poor' live Jesus' teachings and Christians building mega churches and following prosperity gospel. Or most upsetting to my grandma, the Christian holy roller stuff. Or more upsetting to the Christians Mother Mary in the Catholic church.

When I was a kid there was also a huge racial element. WASPs versus Catholic congregations made up of Italians, Irish, Filipinos, Latin Americans, Middle Easterners and Germans.

But from my experience it's largely the 'stay in your lane' thing. I've had people be friendly with me because since I was christian they assumed I was Christian christian, and when they found out I was Catholic they stopped talking to me (past tense as I'm not really religious).