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

I don't imagine there is a particularly simple answer to that.

People who like international space projects are likely to claim
things like (a) they enable a group of nations to take on a project
which done of them could do alone, (b) they cement friendships
between nations (e.g. US-ESA), provide a foot in the door for
potential friendships (e.g. US-Russia especially in the 90's), and
other non-space benefits, and (c) it is harder to cancel a project if
doing so would let down your international partners.

People who don't like them are likely to say (a) the increased costs
as a result of the budgetary uncertainties in each country, need to
coordinate, etc, outweigh any savings from being able to get each
country to pay for part of the mission, and (b) the space program
should not be held hostage to foreign policy objectives.

Apparently those in charge of Area/CEV are of the non-international
thinking, whereas those in the Clinton and Bush I (or was it Reagan?
I don't remember when Europe and Japan were brought in to ISS)
administrations were of the international thinking.