PowerPoint has low information density. It forces the author to communicate using bullet points whcih incorrectly emphasizes (or de-emphasizes) key information.
For example at NASA, the Columbia Accident Investigation Board formed to investigate the tragic accident that killed all seven astronauts, found that presentations given to officials (using PowerPoint) were over-optimistic about the dangers of damage to the wing. This was due to the use of bullet points and slides.
Memo style (long form writing) is better for communicating complicated/complex ideas.