As far as I know the Neptunia franchise I quite popular. With the games selling better in the states than even Japan. Plus there's limited editions, dlc, and other merchandise that would easily give them the money to keep making games.
The games themselves are also fairly simple to make as well, compared to alot of other 7th gen games so it's not impossible for them to just keep cranking them out, since all they need to do is edit tags and diolouge.
What some people may forget is that the game doesn't take itself seriously at all. It's a big joke(much like the Disgaea series) which is why people like them.