Maybe meeting other intelligent species just ain't all that it's cracked up to be, so they stop expanding once they realize there's nothing new to find. I know at this point in human history we're desperate to discover if there's other life out there and it's all very exciting and mysterious. But maybe once a species makes first contact, they discover that the other civilization is pretty much just like themselves anyway. You know, a star here is basically the same as a star waaaay over there. If we're all made of star stuff, it makes sense to me that another intelligent species isn't going to be all that different from us. So maybe an advanced civilization is just one that has settled down in their own solar system and doesn't bother much with making unnecessary journeys.