1. PS3 was extremely hard to develop for. This caused problems with developers and PS3 missed out on some games. Sony wont make this same mistake with PS4. They even starting asking developers for opinions on creating the next gen hardware... and VITA is apparently much easier to develop for too. All rumors suggest PS4 will be more PC-like in it's build, and therefore easier to develop for.
2. PS4 and 720 will more than likely release at roughly the same time (Sept-Nov 2013). Therefore devs wont get more familiar with one console, and leave the other out like they did at the start of this gen. 360 getting a year head start led to exclusive games like Dead Rising and Saints Row.... that wont happen when PS4 and 720 are released at the same time.
3. Bungie is multiplatform now. They made 3 of the 360s best selling exclusives.
4. Valve is now on good terms with Sony since they allowed Steam on PS3. PS3 also had the best looking console version of Portal 2. Left 4 Dead 1 and 2, are two console exclusives that sold well on 360 and generated a lot of hype. MS wont have their exclusive support next gen.
5. EPIC games is being very vocal about what they want on next gen systems. They have much more influence this time around, so Sony will have to listen to them now.... unlike how they ignored EPICs request for more RAM on PS3. UE3 had issues on PS3 that took a long time to figure out, and it probably led to a lot of popular early-gen UE3 games becoming 360 exclusive (BioShock, Mass Effect, Gears of War). It wont happen again next generation.
6. EPIC owns the Gears of War Franchise and can do whatever they want with it (source:
http://www.vg247.com/2010/03/15/rein-epic-owns-gears-ip-can-do-what-we-want-with-it/). Nothing is stopping them from signing with EA or Activision and making Gears multiplatform next gen. They are missing out on millions of sales by not being on PlayStation systems. It would be a HUGE blow to Microsofts next console if Gears is on PS4. Not saying it will happen, but its very possible. Say GEoW: Judgement comes out and they see sales are tailing off a little bit.... EPIC could decide to go multiplat if EA offers them a BF3-like marketing campaign for Gears 4.
When you look at the big picture. All of the exclusives that helped sell the 360 are in different situations now. Halo is made by different developers, who are unproven. BioShock is multiplat. Mass Effect is multiplat. Dead Rising and Saints Row are multiplat. Future VALVE games like Left 4 Dead will be multiplat.