Also, while open source definitely needs some funding, it can become badly harmed by too much of it, when code becomes developed purely for money and not for the usual ESR’s open source incentives.
As for the problem with a paid version - it's because when it happens, shortly afterwards the free version starts to rot. Monetisation might be good for the owner, but it's almost always terrible to user experience.
If I'm not mistaken, they were publishing the exact same version on both app stores, and the apps could be purchased or some of the features were put behind in-app purchases on Google Play. This is a reasonable funding model that has allowed many open source apps to thrive, though in this developer's case maybe it did not meet their needs.