Here in Germany, there were many voices in favour of making the ticket completely free. What I heard about the reason for not doing so is that the government wanted to easily measure the actual demand in different regions for such a "free" ticket. So the price is low enough that anybody who wants to ride practically "for free" is buying it, but so high that most people who do not really use it won't buy it.
I expect that if the ticket is a success, it will be replaced by real free rides in the future. And the results of the test period could help determine how the costs should finally be shared between the different regions and local governments.