It's the slogan of Morton Salt. They came up with it in 1911.

When it rains, it pours describes how after there has been no rain for a long time, it suddenly rains a lot all at once. Example: "It hadn't rained for over two months. Now it has started raining and it has been raining for a week straight." Reply: "When it rains, it pours." When it rains, it pours means that once something happens after a long pause, it happens in large amounts. Example: "It looks like everyone in our department is sick again, and all at the same time." Reply: "When it rains, it pours." When there is a lot of rain all at once we say that it is pouring. Some people say, It never rains, but it pours; the meaning is the same as When it rains, it pours. Example: "Sometimes we have no customers for two or three hours, then suddenly we get 20 people all at once." Reply: "It never rains, but it pours." http://www.goenglish.com/WhenItRainsItPours.asp Similar to "It never rains but it pours" Meaning: When troubles come they come together. Origin A proverbial phrase. The origin is unknown but the phrase itself was known by the early 18th century. For example, this item from a work by John Arbuthnot, 1726: "It cannot rain but it pours; or London strow'd with rarities." http://www.phrases.org.uk/meanings/205100.html "One stroke of good (or ill) fortune is often followed by many other instances of luck (or misfortune) when you least expect them. The proverb dates back to the eighteenth century. In 1726, English physician John Arbuthnot (1667-1735), published a book entitled 'It Cannot Rain But It Pours.' Jonathan Swift (1667-1745) and Alexander Pope (1688-1744) collaborated on an essay entitled 'It Cannot Rain But It Pours.' The saying has been use ever since." From "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996). The saying, in a slightly different form, is the slogan for Morton Salt: ".The company developed a salt that would be free-running even in damp weather. In 1911, a little girl with an umbrella and her now-famous slogan, 'When It Rains It Pours,' were created to promote this new product in a national consumer advertising campaign. The Morton Umbrella Girl and slogan first appeared on the blue package of table salt in 1914. Throughout the years the ageless girl has changed dresses and hairstyles to stay fashionable. She was updated in 1921, 1933, 1941, 1956 and 1968. Together they have symbolized the growth and progress of the company through the years." http://www.phrases.org.uk/bulletin_board/26/messages/423.html An archaic use of but to introduce an inevitable accompanying circumstance. It cannot rain but it pours. [1726 J. Arbuthnot (title)] The singers were the same as I had heard at the Burletta. ‥‘It never rains, but it pours!’ [1770 C. Burney Music, Men and Manners in France and Italy (1974) 22 July 54] A wife with a large fortune too. It never rains but it pours, does it, Mr. Thorne? [1857 Trollope Barchester Towers III. xii.] I listened to the radio. Ben Gurion had suffered a stroke. ‥It never rains but it pours. [1979 L. Barnea Reported Missing vii.] When it rains, it pours, and Matt Sullivan is being flooded out by women who want to have sex with him. [2002 Washington Post 1 Mar. C5] When something occurs it often does so to excess. For example, First Aunt Sue said she and Uncle Harry were coming for the weekend and then my sister and her children said they were coming too--it never rains but it pours. This expression may have come from either a book by Queen Anne's physician, John Arbuthnot, or an article by Jonathan Swift, both entitled It Cannot Rain But It Pours and both published in 1726. http://www.answers.com/topic/it-never-rains-but-it-pours