Ah ok.
Could be lots of reasons why they didn’t download. Maybe they heard about the promo, but didn’t know exactly what it involved – the searched, you came up high in the rankings so they chose your page, then they went to the page but realised it wasn’t what they wanted after all.
Or something else put them off – didn’t like the site, didn’t like something they read. Or as LadyH said, they just got distracted.
Be glad that they found your site in the first place, at least you’re ranking higher (or better) than your affiliate competitors 