Gee. I wonder why the Clark/Clinton team did this.
The real question is why now?
The answer is that Kerry is supposed to capture the remaining necessary delegates and win the nomination when New York, California, Massachusetts, Florida, and Texas go to the polls on March 2 and March 9.
They had to derail Kerry before then or he'd lock up the nomination. If he falters now and no nominee wins a majority of delegates, then all delegates are freed from their commitments and can vote for anyone in second and subsequent rounds of nominating.
-PJ