Welcome to the forum, nineoaks!
I think the reason minimart hasn't been accepted up till now is that it is often written with a hyphen, mini-mart, or even as two separate words, mini mart (although this version would lead me to expect an establishment selling short skirts or small British cars).
Some dictionaries have minimart, some have mini-mart and some have neither. Some British dictionaries label it as North American, but whatever its origin, the use of the term seems to be global nowadays. Actually, the earliest OED citation for mini-mart is from Kingston Jamaica in 1959!
It seems to me that the hyphenated version is used a bit more often than the single-word form, especially in the UK. However, minimart is well-enough established that it should be allowed in the future. Of course, it might be some time before the letters come up again!
