Take 2 2007-2008 Release list reveals GTA IV DLC 360 exclusive still...

[Dolla Dolla]

I think it's because the way Sony's network is setup, Take 2 has to provide a different means of distribution. Microsoft and Sony have different structures, and it is probably a bit easier and less costly to do downloadable content w/Microsoft.

Edit: Not to say Sony won't get it, it just might take longer and cost Take 2 some extra coin.