It's no different than the constant appeals to Reagan on the GOP side. Although I would add that politics has always been like this, or rather always gone through cycles such as this.
Republicans are so bereft of any popular figures in recent political history that Reagan is pretty much "it". Before that, you have Teddy and Lincoln, and the latter was the first Republican. Teddy ran third party, and ushered in Wilson. Good job.
No one's pushing for a Nixon, Bush, or Ford revival, nor is there a doctrine surrounding them. Democrats have a wider field from which to draw their heroes.