It's really because of Apple and their rules. Apple discourages Free and Pro versions of the same app (they may have loosened that up recently, but at one time, in an effort to combat app clutter and promote IAP, they would reject you). On iOS, there is no real way to try-and-buy without Freemium. At least in Android, and I think WP, you can get a refund within a certain time frame. Couple this with users generally seeking out free or super cheap apps and treating them as fairly disposable, just having a premium app for an upfront price, not being an established known brand, is basically a death wish.

I do this exact same thing