Spoken words require more attention to sound. For instance, you probably would never say “eschew” from the podium — it sounds too stuffy and almost obscene — but you could use in in a written piece. Some word combinations — fluent French for example, or bright blue — can be difficult to pronounce, especially when you’re nervous and your mouth is dry. (That’s one reason it’s always wise to practice your speech out loud: to discover — and revise — sounds that might give you trouble before you’re in front of a crowd.
Spoken words need to be conversational. You can get away with more formal usage when you’re writing. But when you’re giving a talk, people expect you to sound somewhat as you sound when you speaking to them in a conversation.
Spoken words must be immediately or almost immediately understood. If you use a word in writing that readers don’t understand, they can think about it for a moment, try to figure it out from the context, or perhaps look it up, then resume reading. But if you use a word that listeners don’t understand, they stop listening to you to try to figure it out. They stop listening; you keep speaking. When they figure it out — if they do — they tune back in to what you’re saying only to find that they’ve missed something.