After doing Episodes IV-VI he had to do I-III. Especially considering he didn't know if he'd do a third trilogy, he had to at least complete the first six.
Plus I like the idea of a different generation in each trilogy.
Meanwhile, after JJ said he'd wrap up the saga with Ep IX, KK is already looking ahead:
Speaking during an interview with The Star Wars Show, Lucasfilm President Kathleen Kennedy stated the franchise will continue with its newly-introduced characters - specifically mentioning Rey, Finn, Poe and BB-8, so make of that what you will - well beyond Episode IX, and that the studio is currently "looking at the next 10 years" of Star Wars films and spinoffs.