I think its a combination of a few things. Their on page content optimization is great and each merchant landing page is very well targeted. They have great domain authority and are known as a comprehensive source of coupons for the merchants they work with. They promote user interaction with voting and comment mechanisms which increases time on site and decreases bounce rate. Their page also loads incredibly fast and they load in non essential user content after the dom has been downloaded. Not too familiar with their linking strategies but I bet they are pretty good. Basically they have a huge team who focuses on it all day long.
I don't know anything about Google's current algorithm, but I would be surprised if they don't factor click-through and other user "satisfaction" metrics into page ranking. Someone searches for XYZ coupon, clicks the RetailMeNot link and doesn't immediately hit "back", RetailMeNot wins some PageRank.
That is definitely a ranking factor although not influencing pagerank.
I think that coupon codes get lots of natural links and sharing because they drive immediate value.
Also, by updating their content constantly to be fresh, that also helps. Google definitely has a way to see which site has the freshest coupons.A site like RMN with the freshest/most recent updates might rank higher.
