... because the likelihood of those expected next-generation games that were not seen today at Sony's press conference just shot up in chances that they will be seen when Microsoft's console is unveiled.
I missed half of the conference due to work, so excuse any mistakes I make in my following predictions.
The two biggest names that were MIA in the conference that come to mind are EA and Epic Games. EA pushed hard for the PS3 during the lead-up to it's release, but found more success on the 360. This time around EA might take their big swings on the Xbox platform. While people are currently pretty cold on any of EA's offerings, it's hard to deny that they do have a stable of potential content to release next generation. DICE has Battlefield, Criterion has Need for Speed (but more hopefully Burnout), and Bioware has whatever RPG they're working on. It would be a good opportunity for Microsoft to jump in and get the mindset that these games are "Xbox games," like the perception Call of Duty has. Then there is also EA's publishing deal with Respawn Entertainment, who's heads have good relations with Microsoft. It's been a couple of years since the formation of that studio, so they could have something to show.
Then there is Epic Games. Epic has already announced Fortnite (currently PC exclusive), and there has been talk about Epic Games getting out of game development and solely focusing on supporting Unreal Engine 4. That is all hearsay at this point, but lets assume it's true. After all the problems UE3 had on the PS3, I would have thought Sony would make it a priority to come out and say the next generation Unreal Engine will work extremely well on the PS4. This makes the complete absence of Epic a bit suspect. I think Epic would be foolish to not have their engine work well on PS4, but I can imagine an instance where they go out on stage during Microsoft's conference and tout how UE4 is particularly optimized to work on the next Xbox.
Of course Sony has E3 to show more games, but still it's fun to speculate what Microsoft has up their sleeve. What else is there to do until the new console are released? Play current games?!