Actually, when you contract the words “??? ???,” it’s not difficult to hear how the “m” could have become an “n.” It’s much more difficult to say “yomtif” than “yontif.” There are people who do say “yomtif,” but it’s not so common because it’s harder on the mouth.
By the way, there are examples of words in the English language that are actually written the way they are to make pronunciation easier. For example, the word “irresponsible” is not “inresponsible” because it’s harder to say. Same for “immobilize” and the like. Obviously ???? ??? isn’t going to have exceptions like that (or at least I think not-correct me if I’m wrong), but you could understand why words change. (Shaleshudis is the same thing- it’s not easy to say “??? ??????,” especially as Americans who pronounce everything ??????, when most Hebrew words are ?????.)