How to enable horizontal view by screen tilt in Windows Phone 8 on the Nokia Lumia 720?
Windows phone has a tile based interaction and for good interactivity,it doesn't allows horizontal tilt view in some areas.Even app developer are given guidelines about it and they build apps according to it.
See the article in the link below -
Quickstart: Screen orientation for Windows Phone
The Start menu and applist do not have a horizontal mode. If something does have one it will do so by rotating the device while holding it like you would if taking a picture of the horizon.