The Meaning of Life, the Universe, and Everything.
Visually, it seems as if armor can only go up until your armor bar is full, but I don't know whether that is actually true or not. On the map I'm making, the 'scope' of battle with damage and health goes up crazy high, and including much higher armor in that equation would be ideal, rather than JUST having the +1000 health points from your equipment.
I really want to include the Porkchops of Resilience, which provide 20 armor when put in the offhand, but I really don't care much for it if armor can't stack higher.
Of course, it can go higher with mods, but that's different and doesn't really work here.