NASA's DSN is old and rusting, while the ESA's antennnas are underused
The ESA has recent very good, reliable antennas for Deep Space Communication.
They are not used at full capacity.
NASA's DSN is more than 40 years old, rusting, saturated, needing repair and not correctly funded.
MRO alone, for instance, should send 4 terabytes of data (10 to 20 times more what any of the older Mars probe has sent so far).
And there are probes everywhere in the solar system!
There is a reciprocity agreement between ESA and NASA; ESA does use the NASA installations but the inverse is not true.
Why? A matter of pride?