Ads are less of a game feature request and more of a business model one. Unfortunately (or fortunately) King doesn't rely on ads like Rovio does.
Firstly I guess that there aren't internal requests for this and we as free users don't have much saying.
Secondly there are a lot of ad providers each with a SDK. They could choose one or two like other closed engines but I presume they want us to be able to choose anything.
Thirdly each SDK adds up to the size of the build. They want to keep it low.
Fourth.. Defold doesn't support videos so no rewarded ads.
As an ending note, ads aren't going away. Brands are moving into the mobile ad space so from now on, ad revenues are only to go up so it would be nice to have options.
Edit: If I leave my engineer cap at home, and think like a business, then having ads available (any ads, doesn't matter which) is a huge boon; there can only be 2 states(has ads or not). Once it has then we can start thinking about how many or what types.
Edit 2: One solution I saw is implementing just one mediation SDK so the mediation provider handles ad providers on the web side of things.