The reason for this is mainly operational, rather than to do with the laws of physics. Radio spectrum is a very scarce resource, and is managed independently by each country's regulatory authority. In order to allocate spectrum to a mobile operator, the national regulatory authority has to make sure that spectrum is not being used by any other services. The spectrum that an operator would like to use for its cellular services might already be in use in that country for other purposes - radar, military communications, RFID(as mentioned in the previous answer), so the regulator has to carefully manage the spectrum to avoid conflict.
Another factor is that the spectrum required to build a modern cellular system has to be in reasonably sized contiguous chunks in order for the system to function (e.g. a spare free 50KHz of bandwidth here and there is no use!). This puts further constraints on the regulators for allocating the spectrum.
The result of all this is a rather inhomogenous country-dependent allocation of spectrum for mobile services. If you now ask about a particular technology, such as LTE, a further complication is that a mobile operator may already be using some of the LTE spectrum for another radio technology (such as WCDMA), and may not consider it economically justifiable to switch to LTE at this time.
So the reasons are regulatory/technological/economic rather than physical. As far as the laws of physics go, there is a relatively small bandwidth of spectrum which has suitable radio propagation properties for cellular communications (reasonable distances, in-building penetration), so there is enormous competition for this spectrum!