Reagan's popularity has risen over time for the same reason Kennedy's popularity has risen over time--nostalgia for the past, and selective memory on the part of the public. That is universal. Reagan is showing huge poll numbers now because he just died, and the media hype has been enormous. And among tv news networks, the picture being painted is incredibly distorted in a way that the print media response to his death hasn't been. But most people don't take the time that some of us do, to read the Washington Post, the NY & LA Times, the Boston Globe, etc. for a balancing of the distortions of tv news.
It really is that simple. Reagan isn't wildly popular today because of him being great. He is just the current focus of our attention.