Actually I have several theories. I will tell you why not too, however. First of all it leaves horrible scars and wounds that last forever. I can show you exactly where any crapemyrtle on earth was topped. It also makes a profusion of smaller branches resulting in a lack of proportion.
All trees have a characteristic shape.
It just so happens that crapemyrtles have one of the most beautiful. Topping does create larger blooms, though fewer of them. And finally it's down right ugly. We don't hack on dogwoods, redbuds, or Japanese maples. This practice may look appropriate behind a chain link fence in a Mississippi trailer park, but I can assure you it is not appropriate for any landscape that you intend to be admired.