Video sales letters work better because they are easier and faster to write. Video is almost instantly perceived as content. As a result,people are more likely to think it's more valuable to pay attention to than simple text is. Most sales letters do give some content, but words for words sake in today's information based sales oriented part of the internet are often perceived as hype.
Long form sales letters will put the guard of people up. They don't begin with a wall up when they see that there is a video to "watch". It is easier to watch and listen sales letter videos rather than reading a long content. People are still not used to being talked to. You control the sales process, if you do not give people a chance to rewind or fast forward the video then you're able to control the sales process.
You're able to deliver your message and then at the end have the order button appear. In a long form letter that attempts to sell, folks will often just scroll down to the bottom of the sales letter, see the price and then leave. You do not always have time to show customers the value of your offer if he / she scrolls down to the bottom to check the prices.
Conversions are higher..so commissions are higher. In most times tripled the conversion of the same material presented in traditional print sales letter. Going through the much simpler and faster process to create similar information that was on a traditional print sales letter in a video audio format will usually increase sales conversions.