It's just really simple when you think about it, last generation GTA was developed for the PS2 first because it was the most popular console and had the higher install base, this gave Sony an advantage with GTA while the PC versions came out at a later date and the Xbox Versions taking upto a year to first arrive. The deals came with the Stories games when Sony contracted them as Sony Exclusive games, thus Liberty City Stories and Vice City Stories were released on the PSP and then later released on the PS2 to squeeze more sales.
Still to this day the Stories games have not been released on PC or Xbox nor have any plans been made for them to be as they will remain Sony exclusives.
This generation things are very different (maybe after all these months I've figured out the real meaning of "Things Will Be Different"?
), Microsoft got a jump start into the market and this team being serious focus on converting developers to program for the Xbox first and even buying out contracts and exclusivity details (which still happens to this day) whilst Sony took their time to release their console, nope, sorry, as they claimed at the time, Their All Round Media Hub and seemed to lose interest in the gaming side of things and wanting to push the media aspects instead (something which they've now finished and actually starting to push games now) but it's because of this twiddling of thumbs that the PS3 version gets nothing. After Sony showed no interest in GTA this allowed Microsoft to carry on doing what they want.
So now GTA IV is released as a Multiplatform Game and Microsoft were able to offer a large sum of money to get some exclusive goodies and make sure they stay on their console. This is the DLC, Part 1: The Lost and Damned and Part 2: *Currently Unknown*, See what people forget, Microsoft have funded the DLC, their money helped create this content. Bitch and moan all you want, but this content will not be appearing on the PS3 in the same way LCS and VCS will not appear on the Xbox.
Tough Luck, Role-Reversed, be happy that Sony have for now aqquired some new exclusive games for the PS3 after learning their very big mistake of this generation, which is, letting Microsoft take over and not keeping a hold of previous exclusive assets.
edit: I realise what I just said may upset and offend some certain users, but I'm sorry, that is just my view on the situation. You don't have to agree, I'm not going to argue my point, this is a forum after all where we can post our views on whatever subjects.