User blog comment:Sppople/Sauron vs. Palpatine/@comment-1683193-20140314194212/@comment-143604-20140316022738

There's also the whole guerrilla warfare thing the Alliance was kind of completely built around. Palpatine wouldn't have ever had a single major target to strike- and if he were to spend time in the field, striking at several targets, he'd be putting himself, and the entire Empire as a side-effect, under threat. Even when the Death Star was completed, they didn't know exactly where the Rebels were.

It's also probably worth noting that we're currently in a precarious place with Star Wars EU information, now that the new films are in development and the canon is being overwritten. Not a problem now, just seeding the idea that all this may be for nothing in a year or two, when we find that none of these feats are canon anymore.