Simply that any group of people cut of from others for a large part of the time tends slowly to modify their language. What starts as an accent becomes a dialect and eventually, if the communities do not talk to each other, a different language. Before travel became so common, languages were often separated by oceans or mountain ranges. And, once a language has split off, people tend to hang on to their native language.

It's not really that so many languages were created. If you look at the very basics of written language - the letters, then there are far fewer character sets than you might expect. We have what we term "western" script, cyrillic, arabic, etc. There are far fewer scripts than spoken languages because spoken language developed well before written script, and some of the earlier written scripts died out or were subsumed into more common script. The languages themselves developed mostly in isolation. Language is used for communication, and you only need to be understaood by a minority of people - those in your immediate locality and those who trade regularly with you. Taking Europe as an example, written script was basically introduced by the Romans and any existing script was forgotten, so all western and central European countries use "western" script, derived from Latin. St Cyril introduced written language is Russia and it's protectorates, so they use cyrillic script. It is the same for other areas. If we remain a global economy then we will eventually settle on one language as the preferred one, so as to be properly understood, and that will most likely be English. What makes me laugh is that it will be American English, which is a complete contradiction. Still, our days of being an empire are past, and our language is our legacy.

The bible says (Genesis 11) that humans tried to reach heaven by their own attempts, symbolising this assault on God by the building of a tower. To show them that they could not reach heaven that way, God scattered them and made it so that they could not understand each other...hence creating languages. While this might sound far-fetched, latest scientific research for the development of languages forms a view quite similar to the biblical one ie there was once one language that all languages are descended from. A scattering did indeed occur, sundering some groups earlier from the main group than others. Some walked far, some stayed nearer to the heartland. All languages underwent immense changes due to technology and warfare/trade. Groups that stayed together form the basis of the language families of today (eg Indo European, Semitic, Bantu, Amerindian, Australian, Austronesian etc), which in turn, split into daughter languages.