[ThrowawayR2]

An oversimplified ELI5 version is that simply shrinking the size changes the relative electrical characteristics of the pieces of the circuits which may cause the chip to no longer function correctly. This is partly because of physics, since making components of the circuit smaller and smaller affects its electrical properties and increases unwanted electrical effects, and partly because smaller processes require changes in how the chip is made which, in turn, changes the electrical properties of circuit components even if they are the same shape and size as the original.