The reason this happens is because of a phenomenon known as Jespersen's cycle. Many languages, including English, have gone and are going through a cycle where the original word for negation is lost and replaced by a new word, going through a period where the two negative words co-exist.

Here is an example of how it happened in English:

Old English: ic ne seah
Middle English: I ne saugh nawiht
Early Modern English: I saw not.

Compare with what's going with French right now:

Old French: jeo ne dis
Standard French: je ne dis pas
Colloquial French: je dis pas