Each region has its own backstory and mythology. Every Generation teaches us about the region that is introduced, and as in HG/SS, we learn a little extra about Johto and Sinnoh. But going back to the same region where we already know the back stories would then make you wonder "Why has so much changed here?".
Take Unova, we know its history between the fueding brothers and the legend of the Original Dragon. The 3 Unova dragons have all been captured, so why would we need to go back? And I don't think it would be wise to build a whole new backstory for Unova with totally different Legendary Pokemon.
I think the closest to a revisiting of the older regions will be remakes. Generation 5's story was intentionally drawn out into sequels, unlike Gens 1-4.