Forever, you gotta stop spelling "a part" as "apart". "Apart" is NOT the same as "a part". Grr.
If you read into these things, you might find it is:
Etymology
From French à part; (Latin ad) + part ("part").
The many other words like this one were previously seperate too; in books from the 19th century it's not uncommon to find terms that are now single words to be two words hyphenated. One can always argue that we're supposed to have moved on by now, but the previous way of doing things was never ruled incorrect, it just fell out of use.