I used pinterest and still hate(d) when it popped up on search.

Without too much conjecture I think the problem is search-related web crawlers and users have very different experiences with pinterest. To the web crawler, the information is there and easily accesible. To a user, it might be behind a login, or part of an image description, or a sidenote, or whatever. The page doesn't exactly load with what you are looking for front and center

Additionally, I don't use it through a browser, I use it in an app, so im not logged in on my browser.