At 9:26 in the "Mythology of Star Wars" interview below, Lucas talks about his attempts to create consistency in the Star Wars universe even if it's different from our own world, and he says that the presence of sound in space was meant as one of the rules of the Star Wars universe (as opposed to sound which is non-diegetic, something the audience can hear but characters in that universe wouldn't, like music and narration):
One of the things I struggle for is to create a kind of immaculate realism in a totally unreal and fantasy world. It's a science that I can make up, but once I make up a rule then I have to live with it. ... One of the rules is that there's sound in space. So, there's sound in space, I can't suddenly have spaceships flying around without any sound anymore, because I've already done it. I've established that as one of the rules of my galaxy, and I have to live with that.
Edit: Apparently the people in charge of the new Disney canon were either unaware of this or chose to ignore it, since the canon novel Lords of the Sith features a scene of Darth Vader pursuing another ship which includes the following line on p. 17:
His ship slammed into the gun bubble and the transport, the inability of the vacuum to transmit sound causing the collision to occur in eerie silence.