During a session on notifications at BUILD 2012, Microsoft has revealed exactly why Windows Phone 8 doesn't include a notification center like Android and iOS. Thomas Fennel, a member of the Application Platform team for Windows Phone, was questioned about the lack of a notification center and conceded that they "ran out of time" including it in the final version of Windows Phone 8.
However, there is hope for the feature to be included in a future version of Windows Phone as it looks like the team does want to include such a feature. "It's very very important to me… we get tons of feedback from developers that they want something like that as well. I promise we're thinking very very hard on that one."
The lack of notification center presents a problem in Windows Phone, as toast notifications simply disappear after they are shown, and the only way to know you have something to attend to is a Live Tile that you have to set up on your phone's Start Screen. A notification center would collate all these notifications in the one area, allowing you to check which ones you need to attend to after they have disappeared from your screen.
Chances are Microsoft will bundle a notification center with the next Windows Phone feature update.
Source: The Verge