Based on no scientific evidence of my own, I would imagine they just went away because the trend of those kinds of banner ads simply kind of vanished. I haven't seen very many "Lose belly fat with this one secret tip only single moms know" ads in a while (I think they were all pitching Acai or something). There are definite trends of ad types across the various ad networks, but like any other market looking to hawk the-next-big-thing, I would think they dried up because business was elsewhere.