Email, because it is written speech, requires the participant to make statements or ask questions in writing. Written speech, as we know, is fundamentally different from oral speech. Oral speech comes with all sorts of non-verbal cues that help convey meaning. Oral speech is more nuanced because of intonation. Irony, for example, is easier to detect in oral speech because the speaker will use an ironic tone.
Oral speech is also quite contingent.
The listener provides both verbal and non-verbal cues that the speaker's meaning is or is not getting through. This allows the speaker to go back and rephrase. Written speech, in contrast, evokes no immediate response from the reader. Usually, the writer must assemble and complete all parts of the message and make sure that each part, especially where the message is cumulative, is clear. Written speech tends to be more concise.