Halloween

[Man in Space]

He had [ɑ], but I was on my phone and could not type that.

[Sol717]

Why just in this word, though?.

It is not "just in this word" though. Plenty of other Scots forms exist in AmE varieties; for instance, obsolete Appalachian [ˈɪŋən] "onion" can be compared to Scots ingan "id." (from earlier *[ˈʊnjʊn] with a change of [ʊ] to [ɪ] as in simmer "summer" and [nj] to [ŋ(j)] as in Menzies, traditionally [ˈmɪŋɪs]).

Because Halloween entred American English from Scottish English? I would have thought Irish English a more likely source given the ethnic makeup of the USA.

There was plenty of Scottish settlement in the US, especially in the South and Appalachia; it is just more opaque because the settlers' descendants retain less of their forebears' ethnic consciousness.

Compare, for instance, the common pronunciation of karaoke as /ˌkɛ.ɹiˈoʊ.ki/. NAE speakers could quite easily better approximate the Japanese pronunciation--there is a personal name Cara after all. But Carrie is more common and probably influenced the shift of the first vowel.)

A more parsimonious explanation is that karaoke is /ˌkæɹiˈəʊki/ (etc.) rather than /ˌkæɹəˈəʊki/ because sequences of schwa immediately before a (secondarily-)stressed vowel.are very rare in English (especially in everyday vocabulary); no analogical explanation is needed.