There's also the possibility that, on the inside, those companies already have relationships with other banks and/or declined Goldman's invite.

I would imagine (and clearly, I'm not an IPO underwriter) that it might have something to do with proving they have a stable, profitable model. Twitter is finally on the way there with their ads, less sure about Foursquare.

Companies like Uber and AirBnb already have their business models mapped out, so even though they are orders of magnitude smaller, they are more attractive to IPO underwriters.