POD(s) Where "Ain't" replaces "Isn't"?

What's the most believable way that "ain't" becomes orthodox, while "am not", "isn't", "aren't", "hasn't", and "haven't" are all considered incorrect?
 
The first part is not that hard. One more contraction to be considered standard. The second, though, is difficult. If no other contraction is acceptable, there must be some reason why exactly this one is, and I ain't seein' it. Language rules are not mathematical, but there usually is some degree of consistency. Either contractions are accepted at some level of formality, or they are not.

Perhaps a development where the first person singular has some kind of special status due to politeness forms? This is not terribly outlandish, English lost the second person singular that way. But I'm not knowledgeable enough to speculate what this could look like in detail.
 
Top