Firstly, because people are already pre-SOLD on the idea that your product is of value and they will gladly opt-in to download your product.
You don’t need to do a lot of ‘convincing’ either on your landing page or lead-capture form. All you need to do is write a simple, “Download here, exclusively for the subscribers of the Fire Sale!” and it will suffice!
Furthermore, there is always a theme involved with every Fire Sale. A good theme ensures that the subscribers are all willing to help you out and buy your product (if they are not sold on the idea of the product’s value, at the very least, they will support your REASON for holding the fire sale and buy from you).