The reason sloped armor is so effective is that it increases the effective thickness of the plate, without adding weight to the tank. This plate of armor is only 100mm thick, but because of the way it is angled back, the effective thickness of armor that a shell would have to go through is twice that of the plate.

Why didn't German tanks have sloped armor?

sloped armor limits internal space. German tanks had a huge transmission in the front as well as a big radio unit. There was not much space to move this equipment back to allow for a sloped front. Also the space above the tracks (sponsons) were used to store ammunition in.

Who invented slanted armor?

The Soviet Union was one of the first to make wide use of sloped armour designs. The A-20 prototype is a clear example, but even on the BT series of fast tanks, it's visibly beginning to come into play.