Because there's no need. The universe of Rapture doesn't need to be in the same as one where you do battle with animatronic President's and cities suspended by balloons. As fantastical as Rapture is, its construction was presented realistically as possible as was it's explaination of Plasmids and such. Columbia is just far too zany for my liking.
I can't help it, it's existance just offends me. BioShock is about Rapture as far as I am concerned, first they made an un-necessary sequel and now this Infinite. Basically BioShock is just a label now, a checklist of items and themes to cross off and that's what makes it BioShock. I have no doubt that Infinite will be a good game but it just feels like a betrayal, almost like "Rapture, oh that's nothing compared to what we have here." Because I know that I will never hold a setting or universe in such esteem Rapture.