The only reason is this... There are camps and such that have "rope courses." Now all that is, is this. They have 2 big trees about 20 yards apart. Then about 20 feet up those trees they have a STEEL cable going a cross. And on both those trees are "Staples" or big steel lopes for foot and hand holds to climb up the tree to the cable. Then once you get to the cable you can walk across it(they have ropes hanging down to help keep your balance) to the other side and climb back down. Now they use those "blue steel cables"(probably different name but same thing) for Saftey ropes. Yes they are strong. Yes they have to be reliable to be holding your life in its grasp. But they aren't great for all kinds of weather. Also UV rays could hurt them too just like Nylon. But if they were great for outdoor use and stress and strain 24/7... why wouldn't they use them to string between the trees? For they stay there 24/7.

Thats just what I think, Plus, since it hasn't been tested and if disaster does strike, someone could die. Then You'll wish you paid more for the steel.