The purpose of language is to communicate. The creation of language was the solution to the problem of communication - and this becomes your answer. Different people come up with different solutions to tackle problems, and just as we have different types of swords in the world, and they are still swords, we have many languages.
This is a simple way of looking at it. Of course we may have to add in factors such as evolutionary growth in different parts of our bodies giving us optimal sound in different ranges, or some other biological reasons but that's a bit too complex :)