The key to it is cost and flexibility.
Space Rider is extremely cheap to develop from IXV hardware (around 50 million total; 32 million to CDR have been funded), and will launch for around 35-40 million per flight. DreamChaser will cost at least 100 million per flight, more in the region of 120 million - which is quite okay too comparing the effective payload.
It is extremely likely DreamChaser will be tied into some sort of flight plan regime for ISS though, even for a "DC4EU" version. That means flights from the US, to the US, on a predefined schedule, with orbital operations of experiments typically then also requiring installing them on ISS (like today).
Minor role is also internal politics. ESA after all is dominated by only three national space agencies: DLR of Germany, ASI of Italy and CNES of France.
DC4EU is mostly pushed by DLR, Space Rider is a ASI project - with CNES sitting on the fence. CNES pushes Ariane for everything, ASI Vega - with DLR on the fence. CNES and DLR both push for cooperation with China on manned flights, ASI is kinda sitting that one out. And so on.