There’s been a lot of criticism (including by me) of Trump’s speech for being too long and not thematic enough. Here’s some of the basic thinking behind why the Trump team did it the way they did:
First, they believe that a short, thematic speech is prone to disappear from the news cycle faster than something longer that gives people more to latch on to. They also considered the acceptance speech a rare opportunity to have Trump characterize Biden’s record in his own words. Once you go through everything worth hitting regarding Biden’s past record and his current policy positions, you have a pretty long speech. To avoid being entirely negative — which would obviously be a permission slip for a press to characterize it as hateful and untoward — you need an uplifting ending and uplifting close. That adds to the length.
The acceptance speech ended up being about as long as Trump’s State of the Union addresses. Those addresses typically don’t get raves for their craftsmanship, but generally go over well.
Besides, why leave any air time on the table when there are so few chances during the campaign for this kind of unmediated communication to millions of people? Most actual voters aren’t like pundits who sit down to watch the entire speech; they might catch only a part of it.
Finally, since most of the TV commentators after the speech are going to be hostile regardless of what Trump says, it’s a side-benefit to push them all as close to midnight as possible.