The major reason why many maps show only a limited region, is the amount of data they have to process if they loaded the entire world. They may also have limited traffic, and want to prevent the whole world to overload it.

There are lots of lists that show where you can find maps, resources, and that’s mostly the problem: too much fragmentation of information. Your best bet would be to look in the wiki for more information, or write to the creators of ÖPNV-Karte.