Much of this is down to the same problem that has plagued Armour development right from the start. The compromise between Protection, and Mobility. It is possible to built a Armoured vehicle, that is impregnable to all known weaponry, but it would be unable to move. The more heavily armoured your vehicle, the less manouvrable it becomes.
The Land Rovers trade Armour Protection for Mobility, relying on speed and fire power to see them through. The Land Rovers are mainly used for fast reconn, rather than slow convoy duty or street patrols.
A Land Rover makes a very hard to hit target when it's on the move. As for protection against IED's and such, they can be made so big/effective, that even Main Battle Tanks are vulnrable.
Henk