the review process is much more careful than facebook or google. they only really want content that would make sense organically on pinterest.
facebook and google are pretty loose about the "salesy-ness" of ads, but pinterest is pretty strict in comparison.
obviously with all these restrictions, agencies don't like it because they have to cater their message to pinterest instead copy and pasting the same noise on every platform.
pinterest probably won't kill google proper, but I could easily see pinterest killing google images.
When is the last time a company seriously decided to take on image search at google scale? Google doesn't even show ads in it's image search.
another reason agencies probably don't like pinterest is the high fixed-floor bids of 10 cents.
you could have lower effective cpcs, but your ads actually have to be engaging, and they have to get repins and shares.
bottom line: Pinterest simply demands more effort from advertisers, and they're pretty picky. Consequently, users don't mind their ads as much.
The best case scenario for pinterest's investors (in my mind) is an acquisition by facebook as part of a strategy to take on the web search market.