Are you talking about of between Episode III and Now what is called Episode IV but what i call Star Wars ?

The reason that is forbidden is because that is the era Lucas is doing the live action series.

Not exclusively forbidden if you count the dark times comics, that other Vader comic that just ended, force unleashed, and dark lord novel the rise of Darth Vader.

But stuff they have not been allowed to to do is what now. They established the rebellion twice once not counted because it was a deleted scene in revenge of the sith, and the second time on force unleashed.