The old ironclads of the Union and Confederacy had slopes, and seeing how in WW2, the T34 had quite an effective sloping armor, I was wondering what were the fundamental issues (be they scientific or not) that dissuaded modern navies from having them post 1860s.
Less a historian's perspective, and more that of an engineer, but the problem with sloped armour is that it takes up interior space. Boxes are incredibly efficient in terms of usable internal space, whereas sloped walls in any application mean that you have less usable space.
Consider the difference between a normal bedroom, and one in an attic with a sloped ceiling. The latter has less usable space in most cases, without resorting to having a much larger volume overall.