Because they won't have as many games as Sony and they know that. Sony has almost 32 or more in house devs. MS has 4. MS never has had as many exclusives. They just would blow their money on pointless timed exclusives and DLC. Not actual creation of games.
Sony has made way more IP's and continue to do so. MS says they are making new ones...but for what? Kinect ( I will bet so).
It's not just about the reveal though. It's about their message. Sony's message was to all devs that be free over here, make games the way you want to make them.
MS's was. NFL? Skype? TV? Have you heard of those? I will bet Sony shows more games/interesting games at E3.
I do see MS going the way of the past couple of E3's. They're starting to become like Nintendo. Showing off 3 staples (Halo, Gears, Forza) like Nintendo's (Mario, Zelda, Pokemon), then going Kinect and multimedia heavy. Used to I'd tune in to all 3 E3 shows. Partially to see if the 360 and Wii could tempt me into a purchase, but also because I was curious what the competition was up to. Recently I haven't had that drive. I tune in to see what Sony has in store for the console I own, and then tune in to individual developers shows. Afterwards I read you guy's thoughts on the MS show to see if any interesting new multiplat games were shown, and thoughts on Nintendo to see if any new interesting handheld games were revealed.
Sony pushed their relationship with Developers, and how they worked on the PS4 together. Microsoft was stood there and said, "Look at all these belles and whistles!" I think Sony had the better conference, and a clearer message.
I took this as Sony learning their lesson. The PS3 was notorious for being hard to develop for, especially early in the gen. They also got flack early in the gen for inferior ports, and a lack of quality titles. This time I think they wanted to come out firing on all cylinders. They wanted to tell developers they're 100% supportive of their efforts, let gamers know good games are coming, and let indy developers know they have their back.
I think Microsoft on the other hand is just showing the face they've always wanted to show. They want it to be a family friendly gaming device that can pull Wii numbers. Above that however they want it to be be the only device you need in your living room (outside of a Windows 8 PC). They know they need the hardcore gaming crowd. They'll be the ones that make sure it sees a 7mil launch year, and push AAA sales to over 5 million.
They just want the console to be more than a gaming console. They want it to be what you turn to for everything. They want you to buy music from their stores, DL and watch movies on the X1, watch TV through it (especially if it's subscription based), play games on it, ect. I think the hardcore PC style gaming stuff was to get their foot in the door, and secure the hardcore US gaming audience. Now that they have that they can become what they really wanted to be in the first place. If that sounds like I'm making them sound like a villain, I'm not. It's actually a pretty smart strategy, it's just that they're risking losing the backbone of said strategy, hardcore gamers.