This is worth reading http://en.wikipedia.org/wiki/Star_Wars_sequeIl_trilogy
It chronicles all of Lucas various comments about his plans and ideas for a sequel trilogy since the late 1970s. Sure they are vague and undeveloped. Some are inconsistent with how the OT ended.
But there is one consistent idea. That a sequel trilogy would be take place decades after the OT. That he envisioned from the start making them with the cast when they reached their 60s and 70s.
So he always intended to set post Jedi movies decades later and be about the core characters. But clearly the backstory, the prequels was the idea he had develped the most and wanted to finish himself.
Perhaps now after reaching old age himself, the idea of seeing his characters at that point appeals to him again. Being able to mostly enjoy it as part of the audience this time with out the stress.