I imagine there's no reason. Just as in English most countries don't have the article but some do (did). English has dropped the article from Ukraine and Lebanon, and perhaps some others. Some people feel it's disrespectful, in English, to have an article. But I don't suppose it's the same in Arabic. Perhaps a native speaker could inform us?
That's because the s is a sunny or solar letter. For those the L in AL is not pronounced and the first character of the word (without al) becomes doubled.
See The Moon And The Sun: Arabic letters and the definite article "AL" for the details.