People who are better at emailing often overestimate how good other people are at reading emails.
Whenever I write a report I always put the summary first because that is all that anyone who trusts me needs to read. The rest is supporting information and extended arguments in favour of the proposition that I make in the summary together with some notes about why alternatives are no good.
Of course, I never went to business school, I'm an engineer who started as a physicist.
I say this as somebody who used to prefer written communication. But I have come to realize that it is necessary to ask your audience, "Does that make sense?" after every major and minor point, and then listen to them reformulate what you just told them to make sure they actually got it.