In English, the words “he is not” can be contracted to either “he isn’t” or “he’s not”, but not to *”he’sn’t”. (The same applies of course to “I am not”, “you are not”, “she is not”, and so on, but for simplicity’s sake I’ll concentrate on “he is not” here.)
Any speaker of English thus needs to choose one of the two possible contractions every time this construction comes up, if they want to contract it at all.
An Irish ex-colleague of mine once commented that he’d observed that Scottish people had a tendency to prefer the latter (“he’s not”), and having paid attention to the constructions since then I tend to agree with him. (It’s hard to be certain without a corpus of spoken Scottish English.)
I’ve also been mulling over why this might be the case, and I’ve come up with two possible reasons, both involving the Scots language:
(1) In Scots, the construction “isn’t he” isn’t possible. You can only say “is he no” (“is he not”), never *”isna he”. This might spill over into non-questions.
(2) In Scots, the two possible constructions are “he isna” and “he’s no”. The phonetic distance between Scots “he isna” and Scottish English “he isn’t” is somewhat greater than the distance between Scots “he’s no” and English “he’s not”, so in a situation where Scots is frowned upon, the latter construction might survive better.
I’m also wondering whether some speakers of Scottish English dislike all the negatives ending in “-n’t” because they sound rather mumbled compared to their Scots counterparts that end in “-na”, but I haven’t found any evidence for this hypothesis so far.