Because you weren't paying attention? Because it was right there all along.
I've taken US history classes in High school and College, and this fleet of ships was never mentioned in any of the classes or the textbooks. I would have noticed because it greatly clarifies why the Confederates opened fire on the fort. That is something that never made sense to me before, and I had always heard it was attributed to them simply being hot heads that wanted a war.
The story of these warships sent to force Charleston to obey Lincoln is curious because of it's omission. One has to go looking for it to find it, but before one can look, one must first have heard of it.