Might it simply be that people find the app for a use they have, right now? When it works great and solves your immediate problem, what's a few bucks?
Whereas if you have a full function trial period, it solves your immediate need for free and then you already have this mentality "this functionality should be free". Maybe even uninstall the app, erase all its data, and then register for a new trial period next time you need it etc.
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