Speaking is harder in many ways than writing because it is performance. ... Writing is harder in some ways than speaking. Writing must be self contained: there is no body language or vocal emphasis as everything must be in the words themselves. But the ability to revise and edit dozens of times narrows the gap.
Another reason some individuals might find it difficult to write is that written language takes on many different registers depending on the function of the written word. Often, these functions are unrelated to spoken language and can thus be considered 'artificial' to the speaker.