Pinterest is a focused version of Tumblr.
Whereas Pinterest focuses on collecting stuff through photos, Tumblr is a blog platform for expressing collections through multimedia (photos, video, quotes, text).
My most popular Tumblr posts are typically a compelling photo plus one or two insightful sentences. The same goes for Facebook, where pictures produce higher engagement than videos or plain text.
Pinterest is merely carving a niche out of something it knows works really well and focusing on that. Instagram is keeping it equally fun and simplistic with pictures on the iPhone. In a way, Pinterest fulfills the Instagram web browser experience.
People like collecting things and showing friends and followers these interests in the easiest way possible. For Pinterest, a picture is a thousand words.