It probably would have required them to get too close to the moon to keep from being detected. The whole plan was based on the element of surprise. As soon as the Imperials realized that their shield generator had suddenly been blown up... BAM, the entire Rebel fleet shows up at their unprotected doorstep.

Then again, this is another thing about ROTJ that irks me (but only just a little bit). Did Han just suck at coordinating his team or reading a watch? They had only just met resistance when the Rebel fleet showed up. It seemed like the fleet might have given them a tiny bit more time to complete their mission just in case of complications (or, you know, maybe keep in touch with one another to be sure), or maybe Han made a mistake of camping out with Ewoks all night and overslept. Who knows?