Note, I'm not saying there is an effect, but it's not quite so easy to say we should have found it if it exists. First, there is a delay between radiation exposure and cancer risk (up to ten years).
Second, if all population centers experience the same increase in radiation exposure, then it is very difficult to use radiation as an independent variable to measure its impact on cancer.
For instance, it took decades before anyone was able to show that nuclear testing caused an increase in deaths, and that was only possible because the distribution of radioactive isotopes was not uniformly spread across the US -- some states (especially in the midwest) received far more radiation than others (e.g. Western California or Louisiana).
In contrast, if electromagnetic radiation from cell towers is relatively uniformly spread for the same population density (e.g. New York is comparable to Los Angeles, and rural Washington is comparable to rural Missouri), then it will be rather difficult to separate cancer risk from electromagnetic radiation from other risks.