Writing has a different set of strengths and is often epitomized as the language of information. It is permanent, detailed, precise and structured for re-use and understanding. Writing transcends time and space. Contracts, wills, legislative and other legal documents are in writing because of these attributes. The speaker creates, memorizes and delivers the speech in the argot of writing rather than in the idiom of speech.
Delivery is formal and impersonal rather than interactive and personal.
The speech is longer than the audience's attention span or the time allotted. The big picture gets lost. Key points are buried under minutia. Speaker develops a severe case of nerves in anticipation of speaking in a language not normally used for speech. Speaker is unprepared to connect with the audience or respond to its feedback.