There are a few factors which mean that a vehicle destined for certain countries can't be used in others.
Some of these inclued :
Lighting
Emissions
Roll-Over Protection
Impact Protection
Tyre Specifications
Then, you have all the other items which aren't regulatory but are different, such as :
Cooling Systems
Steering Wheel Position
ICE Levels
Trim Levels
In reality, most specs of vehicles *can* be converted for use in most countries, but it can be an expensive and complicated process and as such, most people never bother :-)