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.