Conditions can vary considerably even over a relatively short distance. An example is that one area may have oil seed rape as the main nectar source, yet only a few miles away it might be heather. The former needs a bee that builds up much earlier in the season than the latter. Adaptation takes a little time to achieve. Local bees are also used to the types and strains of diseases prevalent in your location and will not import anything new. Bees imported from a distant location might also be genetically incompatible with your local stocks and this could result in unwanted behaviour in future generations.
for details of recent research see http://bibba.com/local-bees-better/