And, of course, the new classic “Why? Because fuck you, that’s why.”
As to the OP, I have no idea what the first instance of the form is, but it does seem to be popular and a logical construction, answering the implied question “why?” “L’Oreal. [why? or why choose L’Oreal?] Because you’re worth it.”
I kind of envision the marketing meeting where they come up with it:
Boss: okay team, let’s brainstorm our new branding!
Team: hmmmmmm…
Team: …
Boss: Come on! Think! Why should you buy this product?
Team: … um… because…
SusyTeamLead: Because you’re worth it?
Boss: I love it! Let’s go with that!
I always assumed that slogans like that were inspired by George Mallory’s reported answer when he was asked why he chose to climb Mount Everest: “Because it is there.”
Written in 1973 when a social revolution and a new spirit of feminism was in full swing, it seems clear that the phrase could only have been written by a woman. Ilon Specht was a copywriter with McCann Erickson back in 1973. She was thought of as original, unconventional, creative and independent. Fortunately, she worked on the L’Oréal account! Specht was just 23 years old when she broke new ground with an ad that was strictly from a woman’s point of view. It famously ended with the signature phrase: Because I’m Worth It!
But culturally, ad-industry-wise, “because” came from the same geniuses who for many years now have been starting every ad with “Introducing …”, as if the product was an elegant young socialite making her stellar debut into society instead of a disgusting gadget that is supposed to unclog your toilet.
“Introducing … the Veeblefetzer Clog-B-Gone Flush-o-Matic! Why? Because we care. About you. Deeply. Trust us, you’re all we ever think about. To us, you are the Über Gegenstandstheorie of Meinongian ontology rationalizing the very existence of the Clog-B-Gone. We invented it for you. Because we love you. Now buy it.”