So let's see... "All the sudden" is used by a small number of uneducated types in the American South and Midwest and in the northern UK. That's quite a lot of land mass, but still, everyone knows these are the uneducated parts of the English-speaking world. If you search Google for variants of this expression, you'll find that "all the sudden" is the most common, followed by "all of the sudden", and finally "all of a sudden". This is to be expected, since users of the Internet are typically uneducated.