It’s likely because the Italians in America were over 3/4ths from the South of the country (Mezzogiorno) in which the Neapolitan dialect is spoken. In Neapolitan language and in Neapolitan speakers learning English, vowels are often pronounced at the discretion of the speaker, and change frequently. Additionally, some vowels are pronounced differently than in standard Italian, so an ‘o’ may sound more like a ‘ah’ to some English speakers