Why is the Premium membership (and the whole app) not for free?

In the app we use various third party services such as maps, place searching, weather forecast, server housing, which are not for free. If we want to keep offering our app, it is not possible without finances; therefore we need to get the necessary finance through the Premium membership option. By the Premium membership subscription you also support the next development of the app as the development and testing are very time- and resource-consuming.