For anyone wondering why 9€? Not 10€ or 15€ or ... - its consistent with 101 consumer psychology.
First: Its not free, its subsidized by the state. Free would suggest for some "not worth it". Not you, most likely, but this is why some free offers are not taken (seriously). If you want to see what free does, look for the Luxemburg experient: https://www.mobiliteit.lu/en/tickets/free-transport/
Second: Higher price point would make a difference for less subsidy payments for the state, but the psychology is "not even 10 bucks". I know that some will argue that 10 Euros will be hard for low income/poor/out of the system people - but again, this is seen as an experiment and a simple message. The data from the usage will help to understand how one can/might change public transportation on a grand scale.
Third: Its during the summer holidays. People with children in school have to book vacation during that time and so the traffic volume in every year before was less than usual on average. In reverse, a lot of people are on the autobahn and the high speed trains on the first and last days of each school holiday, which are different in each federal state.