How about because they've got good stories? The plot in the Ep. I-III trilogy is actually quite good. There's conflict in many forms adding up richly into complicated entanglements leading to wars. Besides the galactic politics, there are personal stories that hold much promise as well. The forbidden love of Annakin and Padame and comic relief of the droids might be tired themes, but Annakin's fall from grace and Palpatine's rise to power are ripe for some good storytelling.Originally Posted by Ken Fitlike
This trilogy's plot has a lot of promise; what it lacks is decent execution. The relationship of the young couple and comedy from Jar Jar and the droids are both marred by horrendous dialouge. Furthermore, much of the political intrigue isn't given enough time to grow so that more room can be made for CGI of spaceships.
Lucas could've done a lot more with his plot, but instead he opted to create formulaic blockbusters. Consequently, Episodes I and II met with critical opposition. Nothing's really stopping him from making Episode III good but his own urge to pander, however. From the reviews I've read so far, he's finally gotten back to the spirit of Star Wars. I hope that's true.