Horoscopes give the life advice that the readers want to hear. By shifting responsibility onto the stars, the reader can follow their own plans without feeling as though their motives were self-centered. Hey, it's what the star chart recommended!
But what readers want to do, and hence want to hear recommendations for, is not a constant over time. The range in typical mindsets today is way different from those in earlier periods. I doubt there was much horoscope advice in 1974 that nudged readers toward participating in a real estate bubble to get rich quick, although that wouldn't have been surprising advice in 2004. Romantic advice from 1984 would have been phrased to make something happen, while in 2014 it would be geared more toward soaking up attention.
How can we study the changes over time? I couldn't quickly find any sources online, nor any references in Google Scholar that suggest this line of thinking has been pursued in "the literature" before.
Tabloid newspapers would probably be too hard to track down, either online or in real life. Women's magazines are readily available in university library archives, and the entire history of Vogue has been digitally archived. Mass market horoscope books have been published, and would be easy to hunt around for.
I might poke around some of the public libraries and thrift stores in my area. If anyone can point me to online sources, though, I can dig through those as well.