Apparently, Lucas was so enamored of the wind up R2s that he bought them for members of the cast and crew of Star Wars. He also suggested that Kenner produce the toy; Kenner declined, wanting to avoid creating competition for their own figure. However, Kenner Canada did release a carded version of the toy (in Japan, it was sold in a sealed baggie).
I've gotta say -- I agree with George on this one: the wind up R2 is a really cool toy. I love all the detail crammed into this little guy (and he is little -- smaller even than Kenner's R2-D2 action figure). The stickers, the chrome accents, the detailed sculpt... He sure is classy!
My Star Wars collection started as specifically a collection of R2 figures, and I remember how thrilled I was to finally snag an example of the wind up in good condition and with both stickers. I was really getting into the Japanese R2s, and for some reason, I saw this as the ultimate symbol of that particular corner of the hobby. I'm not entirely sure why. Maybe it was the George Lucas thing, or just the strangeness of the figure (because few people can do strange like Japanese toy makers). Maybe I just like that it makes me smile every time I look at it.
...Probably all of the above...