I think if they tried hard enough to make it just PG the movie would have sucked just as bad as the previous two.
It's a trade off...no Ewoks, limited Jar Jar and an appearance by Chewbacca for maybe a little more violence and a darker feel. (which in my opinion as a 30 year old Star Wars geek totally rocked!)
Jar Jar and the Ewoks were horrible. However, there was still plenty of violence in those movies. The violence was just presented less graphicaly. A movie can be dark and moving, while still getting a PG rating.
In episode one someone was cut in half (via special effects).
In episode two many winged humanoids were cut to pieces in close combat, and Skywalker had his arm cut off.
There was plenty of violence, just not lots of gore.
Violence seems to get movies a PG rating. Gore, vulgarity, or nudity appears to be what pushes movies to PG-13 or R. I haven't seen Episode 3 yet, but I wonder why they couldn't keep it PG.