It seems a mystery why they are so "OH EARTH" all the time. But maybe it is because humans have some inferiority complex? Like they assimilate someone, a top science dude, and he's still thinking and being in his heart of hearts that he really doesn't know everything and might actually be a bit of fail. So then the Borg are like, okaaaaaay.. let's go get someone who does know it all. Rinse and repeat.
But you take a Vulcan, or a Klingon and they all like "I AM SUPERIOR" and "I AM THE MAN". So the Borg, who have a zero in psychology, think oh right, got everything we need here, no reason to pursue this race as a whole.
Another aspect of it could be "These bastards keep beating us, so clearly there's more to them than what we've assimilated up to this point. Let's go back and make sure we get it all"
Travelling back in time to wipe us out doesn't cover that. Unless the Borg Queen was a bit more petulant about it of "We are perfection, and I'm tired of you lot proving we're not. So I'm going to make sure you can't!"