edit: Just to be clear, the service in dispute is UberPop.
Anyone can get a taxi license as long as they fulfil some (basic, really) conditions.
To get the taxi license you have to show that you and your car are fit to drive and that you probably will not cause too much damage in the process. That means taking exams, regularly having your car checked (there is a difference between checks for commercial and non-commercial vehicles here) and having insurance that lives up to German standards (an insurance that pays 1 000 000 $ per accident like the one shown on that certificate of Uber's is a joke).
Not that significant a barrier but it keeps out the duds and the crazy. The whole process is easy enough so that supply well meets or exceeds demand in most German cities, even at strange times of day. Maybe Uber is too stupid? I don't think so...
Uber does not want any of that. They only want to have a clear path for the race to the bottom. They do not care about safe transportation, just about their bottom line (I would, too, if I had raised that amount of capital for what is basically a phone app). That's ok, they are for-profit, this is what they're here for. The Frankfurt court has done what it is there for. It has told them that, while rules are in place, they need to be followed. Uber had plenty of time to prove to authorities that they are OK guys. They didn't. They are not "disruptors". By not following regulations, they are, most probably, simply breaking the law and competing unfairly.
Of course you can be cheaper if you skip the background check, driving test and reliable insurance.