When you speak, those in hearing hear your words, and then they’re gone. You speak again, and can choose to say things differently. Thus, the spoken word evolves. New phrases, new pronunciations.

When you write, the written text exists and persists, potentially for a long time. At various times in history, writing has been something that took time or expensive materials, so it was less common to do it for trivial or short-lived purposes. It’s easy to forget in the modern digital age with the disposable, ephemeral nature of Twitter and text messaging, but by its very nature, writing is designed to last. Therefore, it evolves more slowly.

That brutally simplifies a whole field of linguistic research, but it’s an explanation.