I think this is got to be the reason.
It's now feared (according to Ken Timmerman's book Countdown to Crisis) that if Iran used all of the secret nuclear facilities that are now being discovered since it started building them in the early 90s (with lot's of help from A.Q. Khan, for one), they would have enough fissile material by now for 25 nuclear warheads.
The scary thing is that no one really knows. Timmerman's book is a must read, by the way.
Cooperation from Europe on the Iran issue is much more important right now then opening old wounds and wasting political capital on proving the WMD issue.