The reason I put stateless code as the highest priority is it's the easiest to reason about. Stateless logic functions the same whether run normally, in parallel or distributed. It's the easiest to test, since it requires very little setup code. And it's the easiest to scale up, since you just run another copy of it. Once you introduce state, your life gets significantly harder.
I think the reason that novice programmers optimize around code reduction is that it's the easiest of the 4 to spot. The other 3 are much more subtle and subjective and so will require greater experience to spot. But learning those priorities, in that order, has made me a significantly better developer.