CEV & Foreign Participation, Why Not?

So NASA has said "No" to European and other outside participation in CEV/
Ares. Does anyone care to speculate why?

I work on Space Station projects at JSC. I have several amusing stories
to tell about working the the Russians, Germans and Japanese. I bet
they have similar stories to tell about working with Americans. But
overall, it has been rewarding on the personal level to work with them.
I suspect that the big picture level, we were better off by partnering
with ESA, JAXA, Roscomos etc. And I don't just mean that lack of
Shuttle flights.

Were there any official statements about why we declined partnership
with the other nations? Did any Congresscritter make significant
statements on the subject?