It was operated by unqualified individuals; they were DC10 pilots, but not fire pilots, and they hit a mountain when they shouldn't have been remotely close.
They were lucky that no ground troops were on the hillside where they struck the trees.
The DC10 also came with the disadvantage of shutting down the fire traffic area each time it showed up for a drop. It had to have it's own lead aircraft, and it was very limited in the tanker bases available, meaning often much longer distances to the fire, as well as the expense.
It's a great platform that's proven itself, but it's also a very big hammer that's often not the appropriate tool for the job. It's an expensive hammer, but a hammer none the less.