Honest, short answer; we don't know.
There's two reasons Bethesda would make the PlayStation community wait longer than the PC/Xbox community, and that's bribery and porting.
Microsoft has been known to pay companies (such as Infinity Ward) to stall the release of DLCs on other platforms to ensure more people will buy the DLCs on their platform(s). Would they do that with New Vegas? Doubt it, since they really only did it with Infinity Ward since Modern Warfare 2 was so highly anticipated and earned so much money.
The other more likely reason is porting. Sony's PlayStation 3 utilizes a newer architecture than Microsoft uses for both the Xbox 360 and the Games for Windows platforms. Games can release on GFW and Xbox much quicker because the architecture is a bit older, and it's similar on both platforms. Basically making a game on the PC is much easier to port to an Xbox 360 than it is to a PlayStation 3.
Will we have to wait again? We can only wait to see. Hopefully if that is the case though, the wait time will be much shorter.