"Aren't I" vs "Amn't I" [duplicate]

Here’s the relevant extract from ‘The Cambridge Guide to English Usage’:

Historically speaking, both ain’t and aren’t are probably descended from an’t, recorded during the late C17 as the regular contraction. Sound changes of the C18 affected the pronunciation of the vowel "a" before nasal consonants, raising it in some dialects, and lowering and retracting it in others. While ain’t is a product of the first process, aren’t represents the second in terms of British (r-less) pronunciation – though not in general American. If only an’t was still available, it would avoid the grammatical discomfort and provide a nonstigmatized alternative to ain’t.

The NOAD has a note about using aren't, and amn't:

The contraction aren't is used in standard English to mean "am not" in questions, as in "I'm right, aren't I?" Outside of questions, it is incorrect to use aren't to mean "am not" (for example, "I aren't going" is clearly wrong). The nonstandard (although logical) form amn't is restricted to Scottish, Irish, and dialect use.

The reason for this pronunciation is due to the accent of the people from which it originated from, the Cockney accent:

1706, originally a contraction of am not, and in proper use with that sense until it began to be used as a generic contraction for are not, is not, etc., in early 19c. Cockney dialect of London

It was actually originally amn't, as can be seen here:

Origin: 1770–80; variant of amn't (contraction of am not ) by loss of m and raising with compensatory lengthening of "a"

The reason it is in popular usage today can be seen in this excerpt:

popularized by representations of this in Dickens, etc., which led to the word being banished from correct English.

"Ain't" is used because it is more popular, and also, it is easier to pronounce.