I'm not sure why
Chrome's app launcher doesn't have a keyboard shortcut, but it's pretty easy to add one. For Windows XP, right-click the desktop shortcut for Chrome App Launcher and select "Properties", go to the "Shortcut key" field and enter a keyboard shortcut. If you use Windows 7 or 8, press Shift while right-clicking the colorful grid icon from the taskbar, select "Properties", go to the "Shortcut key" field and enter a keyboard shortcut. I picked Ctrl+Shift+A.
Now I can just press Ctrl+Shift+A and the launcher will show up. I can type a Google query, the first letters from a Chrome app or just use the arrow keys to pick an app.
Google now even shows suggestions from the Chrome Web Store, so you can quickly install apps without visiting the store. I wouldn't recommend doing that, unless you really trust the app. For example, you can search for "gmail" and quickly install Gmail Offline.
