It wasn't a bad decision so much as it was a poor throw.
Still, if you watch the replay, in order for Cassell to have thrown it sooner when Williams was open, it would've required a faster developing play. So even if Cassell puts enough arm into the throw, it's going be a close call for real estate in the back of the endzone if Cassell leads Williams correctly.
Merriweather is able to get over because the ball hung in the air, but even if the ball is on the money and leads Williams out in front, it's going to be very close to the back of the endzone. I don't think the throw was late at all, and the replay supports that argument. Cassell threw it as soon as he came out of his playaction so he couldn't have thrown it any sooner than he did.
I just think that play was designed for 55 yards or so, but we attempted it from under 50.