I think it's the difference between paying the word processing department (if those still exist) to edit last year's document and paying for what amounts to an expensive marketing campaign that few people are going to see.
It is mostly a style/taste thing, although I know that some US companies are also interested in improving the presentations of their filings.
There are a lot of limitations and constraints involved when you're filing HTML that needs to look decent against all browsers and operating systems. Some filers will make a highly designed report and file it as a PDF alongside the official HTML version.