Here's why:
* Limited contact: When groups of speakers are geographically separated, they have less contact with other groups speaking the same language. This can lead to independent development of pronunciation, vocabulary, and grammar.
* Influence of local environment: Isolated communities are more likely to be influenced by their local environment, including neighboring languages, regional customs, and even geographical features. These influences can shape the way people speak.
* Lack of standardization: Without constant contact, dialects in isolated regions may evolve without being influenced by standard forms of the language, leading to greater variation.
Example: Consider the development of the American English dialects. The early settlers in different parts of the United States came from different regions of England, each with their own dialect. Over time, these regional differences were amplified by geographic isolation, leading to the distinct dialects we see today, such as Southern American English, New England English, and Mid-Atlantic English.