The only prominent thing to click on from the landing page is the download link, and the ads are only showing to desktop users.

Maybe people don't know how to open it? Some websites exactly show users where to find the download afterwards. With Firefox, downloads pretty much disappear. In Chrome they're a bit more visible but easy to overlook. You could also trigger false positives with antivirus software preventing users to open it. There are a lot of reasons why users might click download but don't install or run the software in the end.

Webscrapers maybe?