I think that's pretty much to be a bit more harsh on Irish people who don't speak Irish, and also to be more recognisable as a title to non-Irish speakers (who

I think that's pretty much to be a bit more harsh on Irish people who don't speak Irish, and also to be more recognisable as a title to non-Irish speakers (who would likely at least still recognise Béarla from years of Irish lessons). "Gan Bearla" or "Without English" would have been an equally short title without resorting to a bilingual title.