The content is simply too old for most people to care about. The newer something is, the more people will be interested in it; the older it gets, the more interest fades. Microsoft knew this, which is why they made the deal for a timed exclusive, so everyone that was really interested in the DLC, would buy the game on the 360. They also knew that people who bought GTA4 on the PS3 would be likely to lose interest by the time DLC for that console came along.
Very odd that there is no information regarding this, whatsoever. Not just that, it's bad form too. Cheaper or not, I'm certain there will be some pissed off people when they discover that the hard drive is smaller than advertised.